Store each hash as its raw 32 bytes in a BLOB column (closes #134)

The head, tail and content columns of the files table are BLOB NOT
NULL and hold the raw 32-byte SHA-256, half the size of the hex text,
in the table and in the files_signature index. The hash phase, the
records, report and trees carry each hash as a [sha256.Size]byte and
compare it with ==; nothing is hex-encoded. Reading a record whose
hash column is not 32 bytes is an error naming its path.

In the tests, testHash is a stand-in hash: a string's bytes padded
with zeros, so fixtures keep the sort order their strings give.

Judgement call: a hash of the wrong length is refused on reading,
not copied silently.

Model: opus-5-5

README.md

@@ -388,9 +388,9 @@ All four subcommands operate on a single SQLite database file:
         size       INTEGER NOT NULL,  -- bytes, from lstat
         mtime      INTEGER NOT NULL,  -- whole Unix seconds of the mtime, from lstat
         mtime_nsec INTEGER NOT NULL,  -- nanoseconds within that second, 0 to 999999999
-        head       TEXT NOT NULL,     -- lowercase-hex SHA-256 of the first 64 KiB
-        tail       TEXT NOT NULL,     -- lowercase-hex SHA-256 of the last 64 KiB
-        content    TEXT NOT NULL      -- lowercase-hex SHA-256; whole file under 100 MiB, else samples
+        head       BLOB NOT NULL,     -- raw 32-byte SHA-256 of the first 64 KiB
+        tail       BLOB NOT NULL,     -- raw 32-byte SHA-256 of the last 64 KiB
+        content    BLOB NOT NULL      -- raw 32-byte SHA-256; whole file under 100 MiB, else samples
     ) WITHOUT ROWID;
     CREATE INDEX files_signature ON files (size, head, tail, content);
     ```

TODO.md

@@ -29,6 +29,12 @@
 
 # Completed Steps
 
+- the `head`, `tail` and `content` columns are BLOBs, each holding the raw 32
+  bytes of a SHA-256, half the size of the hex text they held. The hashes stay
+  raw from the hash phase through the database to `report` and `trees`. Reading
+  a record whose hash is not 32 bytes is an error (2026-10-26,
+  https://git.eeqj.de/sneak/sfdupes/issues/134)
+
 - the hash phase gives every file the walk records three hashes: `head` and
   `tail`, the SHA-256 of its first and last 64 KiB, and `content`, the SHA-256
   of the whole file under 100 MiB and of 1 MiB at each gigabyte from 100 MiB up.

cancel_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"database/sql"
 	"errors"
 	"fmt"
@@ -762,7 +763,9 @@ func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 
 	var hashed atomic.Bool
 
-	hash := func(path string, size int64) (string, string, string, error) {
+	hash := func(path string, size int64) (
+		[sha256.Size]byte, [sha256.Size]byte, [sha256.Size]byte, error,
+	) {
 		hashed.Store(true)
 
 		return hashSignature(path, size)
@@ -802,7 +805,9 @@ func TestHashWorkerAbandonsBlockedSend(t *testing.T) {
 
 	// The scan is cancelled while the worker hashes, so the worker has
 	// already passed the check that drops queued runs.
-	hash := func(path string, size int64) (string, string, string, error) {
+	hash := func(path string, size int64) (
+		[sha256.Size]byte, [sha256.Size]byte, [sha256.Size]byte, error,
+	) {
 		cancel()
 
 		return hashSignature(path, size)

db.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"database/sql"
 	"errors"
 	"fmt"
@@ -48,16 +49,17 @@ const lockFilePerm = 0o600
 // createTableSQL is the schema applied to a fresh database. Paths are
 // BLOBs because Unix paths are raw bytes, not guaranteed UTF-8. mtime
 // holds whole Unix seconds and mtime_nsec the nanoseconds within that
-// second.
+// second. head, tail and content each hold the raw 32 bytes of a
+// SHA-256.
 const createTableSQL = `
 CREATE TABLE files (
     path       BLOB PRIMARY KEY,
     size       INTEGER NOT NULL,
     mtime      INTEGER NOT NULL,
     mtime_nsec INTEGER NOT NULL,
-    head       TEXT NOT NULL,
-    tail       TEXT NOT NULL,
-    content    TEXT NOT NULL
+    head       BLOB NOT NULL,
+    tail       BLOB NOT NULL,
+    content    BLOB NOT NULL
 ) WITHOUT ROWID
 `
 
@@ -92,6 +94,10 @@ var errSchemaVersion = errors.New("unsupported database schema version")
 // database.
 var errScanRunning = errors.New("another scan is running")
 
+// errHashSize reports a record whose head, tail or content is not the
+// 32 bytes of a SHA-256.
+var errHashSize = errors.New("hash is not 32 bytes")
+
 // databasePath resolves the database location: SFDUPES_DATABASE when
 // set and non-empty, the compiled-in default otherwise. It returns the
 // path, with the working directory, its symlinks resolved, put in front
@@ -492,19 +498,24 @@ func loadFileRows(ctx context.Context, db *sql.DB, total func(n int64),
 
 	for rows.Next() {
 		var (
-			path      []byte
-			sec, nsec int64
-			r         scanRec
+			path, head, tail, content []byte
+			sec, nsec                 int64
+			r                         scanRec
 		)
 
-		err = rows.Scan(&path, &r.size, &sec, &nsec, &r.head, &r.tail,
-			&r.content)
+		err = rows.Scan(&path, &r.size, &sec, &nsec, &head, &tail, &content)
 		if err != nil {
 			return fmt.Errorf("read record: %w", err)
 		}
 
 		r.path = string(path)
 		r.mtime = time.Unix(sec, nsec)
+
+		err = r.setHashes(head, tail, content)
+		if err != nil {
+			return fmt.Errorf("read record %s: %w", escapePath(r.path), err)
+		}
+
 		fn(r)
 	}
 
@@ -561,17 +572,22 @@ func loadSignatureRows(ctx context.Context, db *sql.DB,
 
 	for rows.Next() {
 		var (
-			path []byte
-			r    scanRec
+			path, head, tail, content []byte
+			r                         scanRec
 		)
 
-		err = rows.Scan(&r.size, &r.head, &r.tail, &r.content, &path)
+		err = rows.Scan(&r.size, &head, &tail, &content, &path)
 		if err != nil {
 			return fmt.Errorf("read record: %w", err)
 		}
 
 		r.path = string(path)
 
+		err = r.setHashes(head, tail, content)
+		if err != nil {
+			return fmt.Errorf("read record %s: %w", escapePath(r.path), err)
+		}
+
 		err = fn(r)
 		if err != nil {
 			return err
@@ -586,6 +602,22 @@ func loadSignatureRows(ctx context.Context, db *sql.DB,
 	return nil
 }
 
+// setHashes sets r's head, tail and content to the values read from
+// those columns. Each must be the raw 32 bytes of a SHA-256.
+func (r *scanRec) setHashes(head, tail, content []byte) error {
+	for _, b := range [][]byte{head, tail, content} {
+		if len(b) != sha256.Size {
+			return fmt.Errorf("%w: found %d bytes", errHashSize, len(b))
+		}
+	}
+
+	r.head = [sha256.Size]byte(head)
+	r.tail = [sha256.Size]byte(tail)
+	r.content = [sha256.Size]byte(content)
+
+	return nil
+}
+
 // countRecords returns the number of records in the database.
 func countRecords(ctx context.Context, db *sql.DB) (int64, error) {
 	var n int64
@@ -756,7 +788,8 @@ func execUpserts(ctx context.Context, tx *sql.Tx, upserts []scanRec,
 
 	for _, r := range upserts {
 		_, err = st.ExecContext(ctx, []byte(r.path), r.size,
-			r.mtime.Unix(), r.mtime.Nanosecond(), r.head, r.tail, r.content)
+			r.mtime.Unix(), r.mtime.Nanosecond(),
+			r.head[:], r.tail[:], r.content[:])
 		if err != nil {
 			return fmt.Errorf("upsert %s: %w", r.path, err)
 		}

db_test.go

@@ -473,6 +473,43 @@ func TestLoadRowsCountAgreeDuringScan(t *testing.T) {
 	}
 }
 
+// TestLoadRowsRefuseShortHash stores a record whose head is one byte,
+// not the 32 bytes of a SHA-256: loadFileRows and loadSignatureRows
+// must each fail on it with an error naming its path.
+func TestLoadRowsRefuseShortHash(t *testing.T) {
+	t.Parallel()
+
+	db := openTestDB(t)
+
+	err := applyChanges(t.Context(), db, []scanRec{{path: "/a/f"}}, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = db.ExecContext(t.Context(), "UPDATE files SET head = x'00'")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	loads := map[string]func() error{
+		"loadFileRows": func() error {
+			return loadFileRows(t.Context(), db, func(int64) {},
+				func(scanRec) {})
+		},
+		"loadSignatureRows": func() error {
+			return loadSignatureRows(t.Context(), db, func(int64) {},
+				func(scanRec) error { return nil })
+		},
+	}
+
+	for name, load := range loads {
+		err := load()
+		if !errors.Is(err, errHashSize) || !strings.Contains(err.Error(), "/a/f") {
+			t.Errorf("%s: err = %v, want %v naming /a/f", name, err, errHashSize)
+		}
+	}
+}
+
 // TestSignatureRowsNeedNoSort checks that SQLite reads signatureRowsSQL
 // from the files_signature index alone, with nothing to sort: the
 // first record comes at once, so report's read phase counts each
@@ -527,12 +564,13 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 	// written.
 	recs := []scanRec{
 		{
-			size: 2, mtime: time.Unix(20, 999_999_999), head: "h2", tail: "t2",
-			content: "c2", path: "/a/tab\tnew\nline",
+			size: 2, mtime: time.Unix(20, 999_999_999), head: testHash("h2"),
+			tail: testHash("t2"), content: testHash("c2"),
+			path: "/a/tab\tnew\nline",
 		},
 		{
-			size: 1, mtime: time.Unix(10, 0), head: "h1", tail: "t1",
-			content: "c1", path: "/a/x",
+			size: 1, mtime: time.Unix(10, 0), head: testHash("h1"),
+			tail: testHash("t1"), content: testHash("c1"), path: "/a/x",
 		},
 	}
 
@@ -553,8 +591,8 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 	// An upsert for an existing path updates in place; a delete
 	// removes exactly its path.
 	upd := scanRec{
-		size: 3, mtime: time.Unix(30, 0), head: "h3", tail: "t3", content: "c3",
-		path: "/a/x",
+		size: 3, mtime: time.Unix(30, 0), head: testHash("h3"),
+		tail: testHash("t3"), content: testHash("c3"), path: "/a/x",
 	}
 
 	err = applyChanges(t.Context(), db, []scanRec{upd},
@@ -581,8 +619,8 @@ func TestApplyChangesBatching(t *testing.T) {
 	recs := make([]scanRec, 0, n)
 	for i := range n {
 		recs = append(recs, scanRec{
-			size: int64(i), mtime: time.Unix(1, 0), head: "h", tail: "t",
-			path: fmt.Sprintf("/batch/%07d", i),
+			size: int64(i), mtime: time.Unix(1, 0), head: testHash("h"),
+			tail: testHash("t"), path: fmt.Sprintf("/batch/%07d", i),
 		})
 	}
 

info_test.go

@@ -23,11 +23,14 @@ const infoCountLine = "[2/2] count: counting the records, elapsed 0s, " +
 // infoRecs are four records: a duplicate pair, a file of 20 MiB and
 // another small file.
 func infoRecs() []scanRec {
+	h, tl, c, u := testHash("h"), testHash("t"), testHash("c"), testHash("u")
+	bigH, bigT, bigC := testHash("bh"), testHash("bt"), testHash("bc")
+
 	return []scanRec{
-		{size: 5, head: "h", tail: "t", content: "c", path: "/d/a"},
-		{size: 5, head: "h", tail: "t", content: "c", path: "/d/b"},
-		{size: 20 << 20, head: "bh", tail: "bt", content: "bc", path: "/d/big"},
-		{size: 7, head: "u", tail: "u", content: "u", path: "/d/unique"},
+		{size: 5, head: h, tail: tl, content: c, path: "/d/a"},
+		{size: 5, head: h, tail: tl, content: c, path: "/d/b"},
+		{size: 20 << 20, head: bigH, tail: bigT, content: bigC, path: "/d/big"},
+		{size: 7, head: u, tail: u, content: u, path: "/d/unique"},
 	}
 }
 

report.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"database/sql"
 	"fmt"
 	"io"
@@ -24,9 +25,9 @@ const minGroupSize = 2
 type scanRec struct {
 	size    int64
 	mtime   time.Time
-	head    string
-	tail    string
-	content string
+	head    [sha256.Size]byte
+	tail    [sha256.Size]byte
+	content [sha256.Size]byte
 	path    string
 }
 

report_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"database/sql"
 	"errors"
 	"fmt"
@@ -18,14 +19,27 @@ import (
 // awkwardDir is a directory name holding every byte the reports escape.
 const awkwardDir = "/d/\tone\ntwo\rthree\\four"
 
+// testHash returns a stand-in hash for a test record: the bytes of s,
+// then zeros. Stand-ins sort as their strings do, so a test can choose
+// the order in which report reads its records.
+func testHash(s string) [sha256.Size]byte {
+	var h [sha256.Size]byte
+
+	copy(h[:], s)
+
+	return h
+}
+
 // awkwardPairRecs is a duplicate pair in sibling directories /d/A and
 // awkwardDir. A raw tab sorts before "A" but its escaped form `\t`
 // sorts after it, so awkwardDir coming first shows that sorting uses
 // the raw path.
 func awkwardPairRecs() []scanRec {
+	h, tl, c := testHash("h"), testHash("t"), testHash("c")
+
 	return []scanRec{
-		{size: 5, head: "h", tail: "t", content: "c", path: "/d/A/f"},
-		{size: 5, head: "h", tail: "t", content: "c", path: awkwardDir + "/f"},
+		{size: 5, head: h, tail: tl, content: c, path: "/d/A/f"},
+		{size: 5, head: h, tail: tl, content: c, path: awkwardDir + "/f"},
 	}
 }
 
@@ -143,8 +157,8 @@ func TestReportStdoutFailsWhileReading(t *testing.T) {
 	recs := make([]scanRec, 2*ioBufSize/len(dir))
 	for i := range recs {
 		recs[i] = scanRec{
-			size: int64(i/2) + 1, head: "h", tail: "t", content: "c",
-			path: fmt.Sprintf("%s/%d", dir, i),
+			size: int64(i/2) + 1, head: testHash("h"), tail: testHash("t"),
+			content: testHash("c"), path: fmt.Sprintf("%s/%d", dir, i),
 		}
 	}
 
@@ -277,12 +291,14 @@ func TestReadReportOnTerminal(t *testing.T) {
 func TestRunReportsIgnoreInsertionOrder(t *testing.T) {
 	// README §Constraints: identical database contents give identical
 	// output, whatever order the records were inserted in.
+	a, b := testHash("a"), testHash("b")
+
 	recs := append(smokeTreeRecs(), awkwardPairRecs()...)
 	recs = append(recs,
-		scanRec{size: 50, head: "b", tail: "b", content: "b", path: "/y/2"},
-		scanRec{size: 50, head: "b", tail: "b", content: "b", path: "/y/1"},
-		scanRec{size: 50, head: "a", tail: "a", content: "a", path: "/x/2"},
-		scanRec{size: 50, head: "a", tail: "a", content: "a", path: "/x/1"},
+		scanRec{size: 50, head: b, tail: b, content: b, path: "/y/2"},
+		scanRec{size: 50, head: b, tail: b, content: b, path: "/y/1"},
+		scanRec{size: 50, head: a, tail: a, content: a, path: "/x/2"},
+		scanRec{size: 50, head: a, tail: a, content: a, path: "/x/1"},
 	)
 
 	reversed := slices.Clone(recs)
@@ -385,16 +401,19 @@ func TestWarnfEscapes(t *testing.T) {
 func TestDupeGroups(t *testing.T) {
 	t.Parallel()
 
+	h, tl, c := testHash("h"), testHash("t"), testHash("c")
+	bigH, bigT, bigC := testHash("H"), testHash("T"), testHash("C")
+	u := testHash("u")
 	recs := []scanRec{
-		{size: 100, head: "h", tail: "t", content: "c", path: "/z/b"},
-		{size: 100, head: "h", tail: "t", content: "c", path: "/z/a"},
-		{size: 100, head: "h", tail: "t", content: "c", path: "/z/c"},
-		{size: 4000, head: "H", tail: "T", content: "C", path: "/big/2"},
-		{size: 4000, head: "H", tail: "T", content: "C", path: "/big/1"},
+		{size: 100, head: h, tail: tl, content: c, path: "/z/b"},
+		{size: 100, head: h, tail: tl, content: c, path: "/z/a"},
+		{size: 100, head: h, tail: tl, content: c, path: "/z/c"},
+		{size: 4000, head: bigH, tail: bigT, content: bigC, path: "/big/2"},
+		{size: 4000, head: bigH, tail: bigT, content: bigC, path: "/big/1"},
 		// Same size as the /z group but a different head hash.
-		{size: 100, head: "other", tail: "t", content: "c", path: "/z/d"},
+		{size: 100, head: testHash("other"), tail: tl, content: c, path: "/z/d"},
 		// A singleton signature must not form a group.
-		{size: 7, head: "u", tail: "u", content: "u", path: "/lonely"},
+		{size: 7, head: u, tail: u, content: u, path: "/lonely"},
 	}
 
 	groups := dupeGroupsOf(t, recs)
@@ -420,10 +439,11 @@ func TestDupeGroupsContentSeparates(t *testing.T) {
 
 	// Same size, head, and tail: /b differs from the others only in its
 	// content hash, so it joins no group, while /a and /c group.
+	h, tl := testHash("h"), testHash("t")
 	recs := []scanRec{
-		{size: 100, head: "h", tail: "t", content: "c1", path: "/a"},
-		{size: 100, head: "h", tail: "t", content: "c2", path: "/b"},
-		{size: 100, head: "h", tail: "t", content: "c1", path: "/c"},
+		{size: 100, head: h, tail: tl, content: testHash("c1"), path: "/a"},
+		{size: 100, head: h, tail: tl, content: testHash("c2"), path: "/b"},
+		{size: 100, head: h, tail: tl, content: testHash("c1"), path: "/c"},
 	}
 
 	groups := dupeGroupsOf(t, recs)
@@ -442,14 +462,15 @@ func TestDupeGroupsMtimeExcluded(t *testing.T) {
 
 	// mtime is informational only; records differing only in mtime
 	// still group together.
+	h, tl, c := testHash("h"), testHash("t"), testHash("c")
 	recs := []scanRec{
 		{
-			size: 9, mtime: time.Unix(100, 0), head: "h", tail: "t",
-			content: "c", path: "/m/1",
+			size: 9, mtime: time.Unix(100, 0), head: h, tail: tl,
+			content: c, path: "/m/1",
 		},
 		{
-			size: 9, mtime: time.Unix(200, 0), head: "h", tail: "t",
-			content: "c", path: "/m/2",
+			size: 9, mtime: time.Unix(200, 0), head: h, tail: tl,
+			content: c, path: "/m/2",
 		},
 	}
 
@@ -464,11 +485,12 @@ func TestDupeGroupsTieBreak(t *testing.T) {
 
 	// The hashes sort opposite to the first paths, so ordering the
 	// groups by hash instead of by first path fails this test.
+	a, b := testHash("a"), testHash("b")
 	recs := []scanRec{
-		{size: 50, head: "a", tail: "a", content: "a", path: "/beta/2"},
-		{size: 50, head: "a", tail: "a", content: "a", path: "/beta/1"},
-		{size: 50, head: "b", tail: "b", content: "b", path: "/alpha/2"},
-		{size: 50, head: "b", tail: "b", content: "b", path: "/alpha/1"},
+		{size: 50, head: a, tail: a, content: a, path: "/beta/2"},
+		{size: 50, head: a, tail: a, content: a, path: "/beta/1"},
+		{size: 50, head: b, tail: b, content: b, path: "/alpha/2"},
+		{size: 50, head: b, tail: b, content: b, path: "/alpha/1"},
 	}
 
 	groups := dupeGroupsOf(t, recs)
@@ -490,15 +512,16 @@ func TestDupeGroupsOrderedByFirstPath(t *testing.T) {
 	// /z, /k, which is neither the order of their first paths nor its
 	// reverse, and must write them ordered by first path. The group of
 	// size 7 comes after them all, though its first path sorts first.
+	a, b, c, d := testHash("a"), testHash("b"), testHash("c"), testHash("d")
 	recs := []scanRec{
-		{size: 50, head: "a", tail: "a", content: "a", path: "/k/2"},
-		{size: 50, head: "a", tail: "a", content: "a", path: "/k/1"},
-		{size: 50, head: "b", tail: "b", content: "b", path: "/z/1"},
-		{size: 50, head: "b", tail: "b", content: "b", path: "/z/2"},
-		{size: 50, head: "c", tail: "c", content: "c", path: "/a/2"},
-		{size: 50, head: "c", tail: "c", content: "c", path: "/a/1"},
-		{size: 7, head: "d", tail: "d", content: "d", path: "/0/2"},
-		{size: 7, head: "d", tail: "d", content: "d", path: "/0/1"},
+		{size: 50, head: a, tail: a, content: a, path: "/k/2"},
+		{size: 50, head: a, tail: a, content: a, path: "/k/1"},
+		{size: 50, head: b, tail: b, content: b, path: "/z/1"},
+		{size: 50, head: b, tail: b, content: b, path: "/z/2"},
+		{size: 50, head: c, tail: c, content: c, path: "/a/2"},
+		{size: 50, head: c, tail: c, content: c, path: "/a/1"},
+		{size: 7, head: d, tail: d, content: d, path: "/0/2"},
+		{size: 7, head: d, tail: d, content: d, path: "/0/1"},
 	}
 
 	groups := dupeGroupsOf(t, recs)
@@ -517,11 +540,12 @@ func TestDupeGroupsOrderedByFirstPath(t *testing.T) {
 func TestDupeGroupsDeterministic(t *testing.T) {
 	t.Parallel()
 
+	a, b := testHash("a"), testHash("b")
 	recs := []scanRec{
-		{size: 1, head: "a", tail: "a", content: "a", path: "/p/1"},
-		{size: 1, head: "a", tail: "a", content: "a", path: "/p/2"},
-		{size: 2, head: "b", tail: "b", content: "b", path: "/q/1"},
-		{size: 2, head: "b", tail: "b", content: "b", path: "/q/2"},
+		{size: 1, head: a, tail: a, content: a, path: "/p/1"},
+		{size: 1, head: a, tail: a, content: a, path: "/p/2"},
+		{size: 2, head: b, tail: b, content: b, path: "/q/1"},
+		{size: 2, head: b, tail: b, content: b, path: "/q/2"},
 	}
 
 	forward := dupeGroupsOf(t, recs)

scan.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"crypto/sha256"
 	"database/sql"
-	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -1199,9 +1198,9 @@ func inodeOfInfo(fi fs.FileInfo) (uint64, uint64) {
 // hash phase.
 type hashResult struct {
 	run     []fileRec
-	head    string
-	tail    string
-	content string
+	head    [sha256.Size]byte
+	tail    [sha256.Size]byte
+	content [sha256.Size]byte
 	err     error
 }
 
@@ -1221,7 +1220,8 @@ type hashPool struct {
 // hash each run's first path with hash (all paths in a run are hard
 // links to the same inode) and write one result per run.
 func startHashPool(ctx context.Context, runs [][]fileRec, workers int,
-	hash func(path string, size int64) (string, string, string, error),
+	hash func(path string, size int64) (
+		[sha256.Size]byte, [sha256.Size]byte, [sha256.Size]byte, error),
 ) *hashPool {
 	ctx, cancel := context.WithCancel(ctx)
 
@@ -1286,7 +1286,8 @@ func feedHashJobs(ctx context.Context, runs [][]fileRec,
 // the hash of only delays that.
 func hashWorker(ctx context.Context, jobs <-chan []fileRec,
 	results chan<- hashResult,
-	hash func(path string, size int64) (string, string, string, error),
+	hash func(path string, size int64) (
+		[sha256.Size]byte, [sha256.Size]byte, [sha256.Size]byte, error),
 ) {
 	for run := range jobs {
 		if ctx.Err() != nil {
@@ -1337,37 +1338,38 @@ func openRegularFile(path string) (*os.File, error) {
 	return f, nil
 }
 
-// emptyHash is the lowercase-hex SHA-256 of the empty input: the head,
-// tail, and content hash of every zero-length file.
-const emptyHash = "e3b0c44298fc1c149afbf4c8996fb924" +
-	"27ae41e4649b934ca495991b7852b855"
-
 // hashSignature computes the three hashes the hash phase records for a
 // file: head and tail (see hashEnds) and content (see hashContent).
 // With the file size they form its duplicate signature, and two files
 // are duplicates only when all four agree. size is the value recorded
 // when the file was statted; a zero-length file has constant hashes and
-// is never opened.
-func hashSignature(path string, size int64) (string, string, string, error) {
+// is never opened. On an error every hash is left zero.
+func hashSignature(path string, size int64) (
+	[sha256.Size]byte, [sha256.Size]byte, [sha256.Size]byte, error,
+) {
+	var zero [sha256.Size]byte
+
 	if size == 0 {
-		return emptyHash, emptyHash, emptyHash, nil
+		empty := sha256.Sum256(nil)
+
+		return empty, empty, empty, nil
 	}
 
 	f, err := openRegularFile(path)
 	if err != nil {
-		return "", "", "", err
+		return zero, zero, zero, err
 	}
 
 	defer func() { _ = f.Close() }()
 
 	head, tail, err := hashEnds(f, size)
 	if err != nil {
-		return "", "", "", err
+		return zero, zero, zero, err
 	}
 
 	content, err := hashContent(f, size)
 	if err != nil {
-		return "", "", "", err
+		return zero, zero, zero, err
 	}
 
 	return head, tail, content, nil
@@ -1377,31 +1379,32 @@ func hashSignature(path string, size int64) (string, string, string, error) {
 // bytes of f. In a file no larger than headTailWindow, each window is
 // the whole file. Reading fewer bytes than asked means the file shrank
 // between the stat and the hash, and is an error.
-func hashEnds(f *os.File, size int64) (string, string, error) {
+func hashEnds(f *os.File, size int64) (
+	[sha256.Size]byte, [sha256.Size]byte, error,
+) {
+	var zero [sha256.Size]byte
+
 	buf := make([]byte, min(int64(headTailWindow), size))
 
 	_, err := f.ReadAt(buf, 0)
 	if err != nil {
-		return "", "", err
+		return zero, zero, err
 	}
 
-	h := sha256.Sum256(buf)
-	head := hex.EncodeToString(h[:])
+	head := sha256.Sum256(buf)
 
 	_, err = f.ReadAt(buf, size-int64(len(buf)))
 	if err != nil {
-		return "", "", err
+		return zero, zero, err
 	}
 
-	t := sha256.Sum256(buf)
-
-	return head, hex.EncodeToString(t[:]), nil
+	return head, sha256.Sum256(buf), nil
 }
 
 // hashContent returns the content hash of f: the SHA-256 of the whole
 // file when it is smaller than wholeFileMax, or of sampled windows when
 // it is that size or larger.
-func hashContent(f *os.File, size int64) (string, error) {
+func hashContent(f *os.File, size int64) ([sha256.Size]byte, error) {
 	if size >= int64(wholeFileMax) {
 		return hashSamples(f, size)
 	}
@@ -1414,20 +1417,20 @@ func hashContent(f *os.File, size int64) (string, error) {
 // reads. Reading fewer than size bytes means the file shrank between
 // the stat and the hash; that is an error rather than a hash of content
 // that no longer matches the recorded size.
-func hashWhole(f *os.File, size int64) (string, error) {
+func hashWhole(f *os.File, size int64) ([sha256.Size]byte, error) {
 	h := sha256.New()
 
 	n, err := io.Copy(h, io.NewSectionReader(f, 0, size))
 	if err != nil {
-		return "", err
+		return [sha256.Size]byte{}, err
 	}
 
 	if n != size {
-		return "", fmt.Errorf("read %d of %d bytes: %w", n, size,
-			io.ErrUnexpectedEOF)
+		return [sha256.Size]byte{}, fmt.Errorf("read %d of %d bytes: %w",
+			n, size, io.ErrUnexpectedEOF)
 	}
 
-	return hex.EncodeToString(h.Sum(nil)), nil
+	return [sha256.Size]byte(h.Sum(nil)), nil
 }
 
 // hashSamples feeds sampleWindow bytes at each gigabyte-aligned offset
@@ -1437,7 +1440,7 @@ func hashWhole(f *os.File, size int64) (string, error) {
 // without every byte being read. Because size is part of the signature,
 // only files of one size are compared, so their sample boundaries
 // always align.
-func hashSamples(f *os.File, size int64) (string, error) {
+func hashSamples(f *os.File, size int64) ([sha256.Size]byte, error) {
 	h := sha256.New()
 	buf := make([]byte, sampleWindow)
 
@@ -1446,11 +1449,11 @@ func hashSamples(f *os.File, size int64) (string, error) {
 
 		_, err := f.ReadAt(buf[:n], off)
 		if err != nil {
-			return "", err
+			return [sha256.Size]byte{}, err
 		}
 
 		h.Write(buf[:n])
 	}
 
-	return hex.EncodeToString(h.Sum(nil)), nil
+	return [sha256.Size]byte(h.Sum(nil)), nil
 }

scan_test.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"crypto/sha256"
 	"database/sql"
-	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -43,13 +42,6 @@ func writeFile(t *testing.T, dir, name string, data []byte) string {
 	return p
 }
 
-// hexSum returns the lowercase-hex SHA-256 of data.
-func hexSum(data []byte) string {
-	s := sha256.Sum256(data)
-
-	return hex.EncodeToString(s[:])
-}
-
 // pattern returns n bytes of deterministic content seeded by tag.
 func pattern(tag byte, n int) []byte {
 	data := make([]byte, n)
@@ -62,7 +54,9 @@ func pattern(tag byte, n int) []byte {
 
 // sig returns a file's full signature (head, tail, content), failing the
 // test on any error.
-func sig(t *testing.T, path string, size int64) (string, string, string) {
+func sig(t *testing.T, path string, size int64) (
+	[sha256.Size]byte, [sha256.Size]byte, [sha256.Size]byte,
+) {
 	t.Helper()
 
 	head, tail, content, err := hashSignature(path, size)
@@ -100,12 +94,12 @@ func TestHashSignature(t *testing.T) {
 			head, tail, content := sig(t, p, int64(len(c.data)))
 
 			window := min(len(c.data), headTailWindow)
-			wantHead := hexSum(c.data[:window])
-			wantTail := hexSum(c.data[len(c.data)-window:])
-			wantContent := hexSum(c.data)
+			wantHead := sha256.Sum256(c.data[:window])
+			wantTail := sha256.Sum256(c.data[len(c.data)-window:])
+			wantContent := sha256.Sum256(c.data)
 
 			if head != wantHead || tail != wantTail || content != wantContent {
-				t.Errorf("head=%s tail=%s content=%s, want %s %s %s",
+				t.Errorf("head=%x tail=%x content=%x, want %x %x %x",
 					head, tail, content, wantHead, wantTail, wantContent)
 			}
 		})
@@ -170,22 +164,25 @@ func TestHashSignatureErrors(t *testing.T) {
 
 	dir := t.TempDir()
 
-	// A missing file: an error, and every hash left empty.
+	var zero [sha256.Size]byte
+
+	// A missing file: an error, and every hash left zero.
 	head, tail, content, err := hashSignature(filepath.Join(dir, "missing"), 1)
 	if err == nil {
 		t.Error("no error for a missing file")
 	}
 
-	if head != "" || tail != "" || content != "" {
-		t.Errorf("missing file returned hashes: %q %q %q", head, tail, content)
+	if head != zero || tail != zero || content != zero {
+		t.Errorf("missing file returned hashes: %x %x %x", head, tail, content)
 	}
 
 	// A zero-length file has constant hashes and is never opened: even
 	// a missing path succeeds.
+	empty := sha256.Sum256(nil)
+
 	head, tail, content, err = hashSignature(filepath.Join(dir, "missing"), 0)
-	if err != nil ||
-		head != emptyHash || tail != emptyHash || content != emptyHash {
-		t.Errorf("empty: head=%q tail=%q content=%q err=%v, "+
+	if err != nil || head != empty || tail != empty || content != empty {
+		t.Errorf("empty: head=%x tail=%x content=%x err=%v, "+
 			"want constant hashes", head, tail, content, err)
 	}
 
@@ -198,8 +195,8 @@ func TestHashSignatureErrors(t *testing.T) {
 		t.Error("no error when the stat size exceeds the file size")
 	}
 
-	if head != "" || tail != "" || content != "" {
-		t.Errorf("shrunk file returned hashes: %q %q %q", head, tail, content)
+	if head != zero || tail != zero || content != zero {
+		t.Errorf("shrunk file returned hashes: %x %x %x", head, tail, content)
 	}
 }
 
@@ -1211,8 +1208,10 @@ func TestSyncScanUnchangedReuse(t *testing.T) {
 		}
 	}
 
+	sentinel := sha256.Sum256([]byte("sentinel"))
+
 	_, err := db.ExecContext(context.Background(),
-		"UPDATE files SET head = 'sentinel' WHERE path = ?", []byte(a))
+		"UPDATE files SET head = ? WHERE path = ?", sentinel[:], []byte(a))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1223,9 +1222,9 @@ func TestSyncScanUnchangedReuse(t *testing.T) {
 			st)
 	}
 
-	if r := recordByPath(t, dbRecords(t, db), a); r.head != "sentinel" {
-		t.Fatalf("head = %q, want sentinel (file must not be re-read)",
-			r.head)
+	if r := recordByPath(t, dbRecords(t, db), a); r.head != sentinel {
+		t.Fatalf("head = %x, want the sentinel %x (file must not be re-read)",
+			r.head, sentinel)
 	}
 }
 
@@ -1265,13 +1264,56 @@ func assertWholeFileHashed(t *testing.T, db *sql.DB, path string,
 	t.Helper()
 
 	r := recordByPath(t, dbRecords(t, db), path)
-	if want := hexSum(data); r.head != want || r.tail != want ||
+	if want := sha256.Sum256(data); r.head != want || r.tail != want ||
 		r.content != want {
-		t.Fatalf("head, tail, content = %q, %q, %q, want %q for each",
+		t.Fatalf("head, tail, content = %x, %x, %x, want %x for each",
 			r.head, r.tail, r.content, want)
 	}
 }
 
+// TestScanStoresRawHashes checks what the database holds for a file
+// larger than both end windows, whose three hashes all differ: head,
+// tail and content are each a BLOB of the 32 bytes of the SHA-256 the
+// scan computed.
+func TestScanStoresRawHashes(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	db := openTestDB(t)
+	data := pattern(3, 3*headTailWindow)
+	p := writeFile(t, dir, "f.bin", data)
+
+	syncTree(t, db, dir)
+
+	var (
+		blobs               bool
+		head, tail, content []byte
+	)
+
+	err := db.QueryRowContext(t.Context(),
+		"SELECT typeof(head) = 'blob' AND typeof(tail) = 'blob' AND "+
+			"typeof(content) = 'blob', head, tail, content "+
+			"FROM files WHERE path = ?", []byte(p)).
+		Scan(&blobs, &head, &tail, &content)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !blobs {
+		t.Error("head, tail and content are not all BLOBs")
+	}
+
+	wantHead := sha256.Sum256(data[:headTailWindow])
+	wantTail := sha256.Sum256(data[len(data)-headTailWindow:])
+	wantContent := sha256.Sum256(data)
+
+	if !bytes.Equal(head, wantHead[:]) || !bytes.Equal(tail, wantTail[:]) ||
+		!bytes.Equal(content, wantContent[:]) {
+		t.Errorf("head, tail, content = %x, %x, %x, want %x, %x, %x",
+			head, tail, content, wantHead, wantTail, wantContent)
+	}
+}
+
 // TestSyncScanSameSecondRewrite rewrites a file in place at the same
 // size with an mtime later in the same second as the recorded one: the
 // next scan must notice the change and re-hash the file. The operand is
@@ -1567,7 +1609,8 @@ func TestScanHardlinksReadOnce(t *testing.T) {
 	ra := recordByPath(t, recs, a)
 	rb := recordByPath(t, recs, b)
 
-	if ra.head == "" || ra.head != rb.head || ra.tail != rb.tail {
+	if ra.head == [sha256.Size]byte{} || ra.head != rb.head ||
+		ra.tail != rb.tail {
 		t.Fatalf("hardlink hashes differ: %+v vs %+v", ra, rb)
 	}
 

trees.go

@@ -287,7 +287,8 @@ func (b *treeBuilder) finish() (*treeNode, []*treeNode) {
 // excluded.
 func fileEntry(name string, r scanRec) string {
 	return "f\x00" + name + "\x00" + strconv.FormatInt(r.size, 10) +
-		"\x00" + r.head + "\x00" + r.tail + "\x00" + r.content
+		"\x00" + string(r.head[:]) + "\x00" + string(r.tail[:]) +
+		"\x00" + string(r.content[:])
 }
 
 // collectTreeGroups groups directories by digest, counting each one on
@@ -337,7 +338,8 @@ func collectTreeGroups(allDirs []*treeNode, super *treeNode,
 // digest is the SHA-256 of its child entries — files serialized with
 // name and signature, subdirectories with name and recursive digest —
 // sorted byte-lexicographically. Filenames cannot contain NUL or "/",
-// so NUL delimiters are unambiguous.
+// and every hash and digest is 32 bytes, so NUL delimiters are
+// unambiguous.
 func (n *treeNode) computeDigest() {
 	slices.Sort(n.entries)
 

trees_test.go

@@ -10,16 +10,6 @@ import (
 	"time"
 )
 
-// Signature hashes shared by the smoke-test records.
-const (
-	f1Head    = "f1h"
-	f1Tail    = "f1t"
-	f1Content = "f1c"
-	f2Head    = "f2h"
-	f2Tail    = "f2t"
-	f2Content = "f2c"
-)
-
 // treesOpenLine is the plain line of trees' open phase when it opens
 // the database at once; it prints it as the phase starts and as it ends.
 const treesOpenLine = "[1/4] open: opening the database, elapsed 0s, " +
@@ -28,6 +18,9 @@ const treesOpenLine = "[1/4] open: opening the database, elapsed 0s, " +
 // smokeTreeRecs mirrors the README smoke-test tree layout: /d/t1 and
 // /d/t2 are identical, /d/t3 differs from them only by one filename.
 func smokeTreeRecs() []scanRec {
+	f1Head, f1Tail, f1Content := testHash("f1h"), testHash("f1t"), testHash("f1c")
+	f2Head, f2Tail, f2Content := testHash("f2h"), testHash("f2t"), testHash("f2c")
+
 	return []scanRec{
 		{size: 3000, head: f1Head, tail: f1Tail, content: f1Content, path: "/d/t1/f1"},
 		{size: 100, head: f2Head, tail: f2Tail, content: f2Content, path: "/d/t1/sub/f2"},
@@ -153,13 +146,14 @@ func TestTreeNamesSortingBeforeSlash(t *testing.T) {
 
 	for _, top := range []string{"/a", "/c"} {
 		for _, p := range []string{"/b", "/b-x/f", "/b.txt", "/b/f"} {
-			content := "c"
+			content := testHash("c")
 			if p == "/b-x/f" {
-				content = "other"
+				content = testHash("other")
 			}
 
 			recs = append(recs, scanRec{
-				size: 1, head: "h", tail: "t", content: content, path: top + p,
+				size: 1, head: testHash("h"), tail: testHash("t"),
+				content: content, path: top + p,
 			})
 		}
 	}
@@ -332,11 +326,10 @@ func TestTreeDigests(t *testing.T) {
 func TestTreeDigestContentSensitivity(t *testing.T) {
 	t.Parallel()
 
-	const sharedTail = "same"
-
+	same, c := testHash("same"), testHash("c")
 	recs := []scanRec{
-		{size: 10, head: sharedTail, tail: sharedTail, content: "c", path: "/r/a/f"},
-		{size: 10, head: "DIFF", tail: sharedTail, content: "c", path: "/r/b/f"},
+		{size: 10, head: same, tail: same, content: c, path: "/r/a/f"},
+		{size: 10, head: testHash("DIFF"), tail: same, content: c, path: "/r/b/f"},
 	}
 
 	_, dirs := treeOf(t, recs)
@@ -397,9 +390,10 @@ func TestCollectTreeGroupsSiblings(t *testing.T) {
 
 	// Identical sibling dirs share a parent, so their group cannot be
 	// implied by a parent group and must be reported.
+	h, tl, c := testHash("h"), testHash("t"), testHash("c")
 	recs := []scanRec{
-		{size: 10, head: "h", tail: "t", content: "c", path: "/p/x1/f"},
-		{size: 10, head: "h", tail: "t", content: "c", path: "/p/x2/f"},
+		{size: 10, head: h, tail: tl, content: c, path: "/p/x1/f"},
+		{size: 10, head: h, tail: tl, content: c, path: "/p/x2/f"},
 	}
 
 	super, dirs := treeOf(t, recs)
@@ -418,10 +412,11 @@ func TestCollectTreeGroupsDifferingParents(t *testing.T) {
 	// /p/a and /q/b contain identical x subtrees, but /p/a has an
 	// extra file, so the parents' digests differ and the x group must
 	// be reported.
+	h, tl, c, e := testHash("h"), testHash("t"), testHash("c"), testHash("e")
 	recs := []scanRec{
-		{size: 10, head: "h", tail: "t", content: "c", path: "/p/a/x/f"},
-		{size: 99, head: "e", tail: "e", content: "e", path: "/p/a/extra"},
-		{size: 10, head: "h", tail: "t", content: "c", path: "/q/b/x/f"},
+		{size: 10, head: h, tail: tl, content: c, path: "/p/a/x/f"},
+		{size: 99, head: e, tail: e, content: e, path: "/p/a/extra"},
+		{size: 10, head: h, tail: tl, content: c, path: "/q/b/x/f"},
 	}
 
 	super, dirs := treeOf(t, recs)