Harden the (*gorm.DB).Scan guard test (closes #232)

The isRowProducer comment now says it matches method names only and
names the evasion that follows: a repo-local Row or QueryRow helper
returning *gorm.DB gets past it. unguardedScans states method values
(f := db.Scan) as out of scope, with the reason. The 40-file floor is
replaced by a check that every directory under cmd and internal was
walked, so skipping a whole package fails the test. The planted
snippets now cover the struct-field receiver and are all valid Go
inside a wrapper that declares the names they use; the Rows snippet
that could not compile is replaced by the variable form the guard
reports. The stale "one caller" sentence is dropped.

Model: opus-5-5

internal/gormlog/scan_guard_test.go

@@ -14,15 +14,12 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-// minNonTestFiles guards the walk below against passing because it
-// found nothing to look at. The tree held 60 non-test .go files when
-// this was written.
-const minNonTestFiles = 40
-
-// isRowProducer reports whether name is a method that returns a
-// database/sql row handle. GORM's Row and Rows return *sql.Row and
-// *sql.Rows, so Scan on the result of one of them is database/sql's
-// Scan and never (*gorm.DB).Scan.
+// isRowProducer reports whether name is one of the GORM and
+// database/sql methods that return a row handle, whose Scan is
+// database/sql's and not (*gorm.DB).Scan. It matches the method name
+// only and resolves no types, so a repo-local method with one of these
+// names that returns *gorm.DB gets past it: Scan on that method's
+// result is not reported.
 func isRowProducer(name string) bool {
 	switch name {
 	case "Row", "Rows", "QueryRow", "QueryRowContext":
@@ -50,9 +47,14 @@ func receiverIsRowHandle(x ast.Expr) bool {
 }
 
 // unguardedScans returns the position of every Scan call in file whose
-// receiver is not a row handle. It fails closed: a receiver it cannot
-// resolve syntactically — a local variable, a struct field — is
-// reported rather than assumed safe.
+// receiver is not a call to a row producer. It fails closed: any other
+// receiver — a local variable, a struct field, a call to any other
+// method — is reported rather than assumed safe.
+//
+// It sees only calls written x.Scan(...). A method value, f := db.Scan
+// followed by f(&v), is out of scope: Scan is never the called
+// expression there, and nobody writes a query that way by accident,
+// which is the mistake this check exists to catch.
 func unguardedScans(
 	fset *token.FileSet, file *ast.File,
 ) []token.Position {
@@ -111,15 +113,15 @@ func skipDir(name string) bool {
 	}
 }
 
-// walkNonTestGo parses every non-test .go file under root and returns
-// how many it parsed along with every unguarded Scan it found.
-func walkNonTestGo(t *testing.T, root string) (int, []string) {
+// walkNonTestGo parses every non-test .go file under root. It returns
+// the directories, relative to root, it parsed a file in, along with
+// every unguarded Scan it found.
+func walkNonTestGo(t *testing.T, root string) (map[string]bool, []string) {
 	t.Helper()
 
-	var (
-		parsed int
-		hits   []string
-	)
+	walked := map[string]bool{}
+
+	var hits []string
 
 	fset := token.NewFileSet()
 
@@ -147,7 +149,12 @@ func walkNonTestGo(t *testing.T, root string) (int, []string) {
 				return err
 			}
 
-			parsed++
+			dir, err := filepath.Rel(root, filepath.Dir(path))
+			if err != nil {
+				return err
+			}
+
+			walked[dir] = true
 
 			for _, pos := range unguardedScans(fset, file) {
 				hits = append(hits, relPosition(root, pos))
@@ -157,7 +164,7 @@ func walkNonTestGo(t *testing.T, root string) (int, []string) {
 		},
 	))
 
-	return parsed, hits
+	return walked, hits
 }
 
 // isNonTestGo reports whether a file name is Go source this check
@@ -189,19 +196,33 @@ func relPosition(root string, pos token.Position) string {
 // logged with its values interpolated. The package comment states the
 // limit; this fails when someone adds a call site anyway.
 //
-// The current tree has one caller, internal/database/database_test.go,
-// which this check does not govern: it is test-only and its SELECT 1
-// binds nothing.
+// Test files are not governed: what a test binds is fixture data.
 func TestGormScanIsNeverCalledOutsideTests(t *testing.T) {
 	t.Parallel()
 
-	parsed, offenders := walkNonTestGo(t, moduleRoot(t))
+	root := moduleRoot(t)
+	walked, offenders := walkNonTestGo(t, root)
+
+	// Every directory directly under cmd and internal is a package with
+	// non-test code, so one the walk parsed nothing in was skipped, and
+	// a Scan there would pass unseen.
+	for _, parent := range []string{"cmd", "internal"} {
+		entries, err := os.ReadDir(filepath.Join(root, parent))
+		require.NoError(t, err)
+
+		for _, entry := range entries {
+			if !entry.IsDir() {
+				continue
+			}
+
+			dir := filepath.Join(parent, entry.Name())
+			require.True(
+				t, walked[dir],
+				"the walk parsed no non-test .go file in %s", dir,
+			)
+		}
+	}
 
-	require.GreaterOrEqual(
-		t, parsed, minNonTestFiles,
-		"parsed %d non-test .go files, so this check found "+
-			"nothing to look at", parsed,
-	)
 	require.Empty(
 		t, offenders,
 		"Scan called on a receiver this check cannot show is a "+
@@ -222,18 +243,45 @@ type scanGuardCase struct {
 	want int
 }
 
+// scanGuardCases covers each receiver form unguardedScans names, plus
+// the row producers it lets through. Each body is valid Go inside
+// plantedFile.
 func scanGuardCases() []scanGuardCase {
 	return []scanGuardCase{
-		{"gorm chain", `db.DB().Raw("SELECT 1").Scan(&v)`, 1},
-		{"gorm receiver", `gdb.Scan(&v)`, 1},
-		{"gorm via variable", "q := gdb.Raw(\"x\")\nq.Scan(&v)", 1},
-		{"gorm model chain", `gdb.Model(&x).Scan(&v)`, 1},
-		{"sql row", `gdb.Raw("SELECT 1").Row().Scan(&v)`, 0},
-		{"sql rows", `gdb.Raw("SELECT 1").Rows().Scan(&v)`, 0},
+		{"local variable", "q := gdb.Raw(\"SELECT 1\")\n\tq.Scan(&v)", 1},
+		{"struct field", `s.db.Scan(&v)`, 1},
+		{"gorm chain", `gdb.Raw("SELECT 1").Scan(&v)`, 1},
+		{
+			"sql rows in a variable",
+			"rows, _ := gdb.Raw(\"SELECT 1\").Rows()\n\trows.Scan(&v)",
+			1,
+		},
+		{"gorm Row", `gdb.Raw("SELECT 1").Row().Scan(&v)`, 0},
+		{"sql QueryRow", `sqlDB.QueryRow("SELECT 1").Scan(&v)`, 0},
 		{"unrelated call", `gdb.Find(&v)`, 0},
 	}
 }
 
+// plantedFile wraps one case body in a function that declares every
+// name the bodies use, so each body is the Go it stands for. The result
+// is parsed, never compiled.
+const plantedFile = `package p
+
+import (
+	"database/sql"
+
+	"gorm.io/gorm"
+)
+
+type store struct{ db *gorm.DB }
+
+func f(gdb *gorm.DB, sqlDB *sql.DB, s store) {
+	var v int
+
+	%s
+}
+`
+
 // TestScanGuard_ReportsPlantedCalls proves the check fires. Without it
 // a detector that matched nothing would satisfy the walk above no
 // matter what the tree contained.
@@ -245,9 +293,7 @@ func TestScanGuard_ReportsPlantedCalls(t *testing.T) {
 			t.Parallel()
 
 			fset := token.NewFileSet()
-			src := fmt.Sprintf(
-				"package p\n\nfunc f() {\n\t%s\n}\n", tc.body,
-			)
+			src := fmt.Sprintf(plantedFile, tc.body)
 
 			file, err := parser.ParseFile(
 				fset, tc.name+".go", src, 0,