Resolve scan operands to their real paths (closes #144)

scan now resolves every operand with its symlinks followed before the
walk, and uses that real path for pruning, walking, records and
warnings. A relative operand is joined to the working directory as the
kernel reports it, so a ".." after a symlink counts from its target, as
with realpath. A symlink operand is scanned at its target instead of
skipped. An operand given through a symlink below another operand, to a
directory outside it, is walked on its own instead of dropped with its
records deleted. An operand that cannot be resolved is a fatal error
naming the operand as given. The walk itself still follows no symlink.

Model: opus-5-5

scan.go

@@ -18,6 +18,8 @@ import (
 	"sync"
 	"syscall"
 	"time"
+
+	"golang.org/x/sys/unix"
 )
 
 // The hash phase gives every file it reads three hashes (see
@@ -209,29 +211,43 @@ func interrupted(walked int) error {
 	return errInterrupted
 }
 
-// resolveRoots converts each PATH operand to an absolute, lexically
-// cleaned path (symlinks are not resolved) and verifies that it
-// exists. Database records are keyed by absolute path, so scan results
-// must not depend on the working directory.
+// resolveRoots converts each PATH operand to its real path: absolute,
+// with every symlink in it followed. Database records are keyed by
+// that path, so scan results depend neither on the working directory
+// nor on the symlinks an operand was given through. An operand that
+// cannot be resolved, because it or a symlink's target does not exist
+// or cannot be read, is a fatal error before any scanning.
 func resolveRoots(roots []string) ([]string, error) {
-	abs := make([]string, 0, len(roots))
+	resolved := make([]string, 0, len(roots))
 
 	for _, root := range roots {
-		a, err := filepath.Abs(root)
+		a := root
+		if !filepath.IsAbs(a) {
+			// The working directory as the kernel reports it, with no
+			// symlink in it. os.Getwd may return PWD instead, which can
+			// name it through a symlink.
+			wd, err := unix.Getwd()
+			if err != nil {
+				return nil, fmt.Errorf("resolve %s: %w", root, err)
+			}
+
+			// Not filepath.Abs or filepath.Join: they clean the path,
+			// which drops each ".." together with the directory name
+			// before it. EvalSymlinks follows a symlink before it
+			// applies the ".." after it, as realpath does. The working
+			// directory ends in "/" only when it is the root.
+			a = strings.TrimSuffix(wd, "/") + "/" + root
+		}
+
+		r, err := filepath.EvalSymlinks(a)
 		if err != nil {
 			return nil, fmt.Errorf("resolve %s: %w", root, err)
 		}
 
-		// A nonexistent operand is a fatal error before any scanning.
-		_, err = os.Lstat(a)
-		if err != nil {
-			return nil, err
-		}
-
-		abs = append(abs, a)
+		resolved = append(resolved, r)
 	}
 
-	return abs, nil
+	return resolved, nil
 }
 
 // pruneRoots drops operands already covered by another operand:
@@ -417,14 +433,16 @@ func (s *scanState) runPhases(ctx context.Context, roots []string,
 
 // walkableRoots returns the operands the walk can start from: regular
 // files, and directories not named .zfs, none of them inside a
-// directory named .zfs. Every other operand is warned about, counted as
-// skipped, and dropped. A dropped operand is no longer a root, so the
-// records stored beneath it are treated as if it had not been given:
-// those under no other root count as outside the roots and are not
-// deleted as unverified. An operand on a filesystem in rules that is not
-// persistent is dropped the same way, with a line saying so that is not
-// a warning, and is not counted as skipped. An operand that fails lstat
-// here is kept, and the walk warns about it unless it no longer exists.
+// directory named .zfs. The operands are real paths from resolveRoots,
+// so these checks apply to what a symlink operand points at. Every
+// other operand is warned about, counted as skipped, and dropped. A
+// dropped operand is no longer a root, so the records stored beneath it
+// are treated as if it had not been given: those under no other root
+// count as outside the roots and are not deleted as unverified. An
+// operand on a filesystem in rules that is not persistent is dropped
+// the same way, with a line saying so that is not a warning, and is not
+// counted as skipped. An operand that fails lstat here is kept, and the
+// walk warns about it unless it no longer exists.
 func (s *scanState) walkableRoots(roots []string, rules walkRules) []string {
 	kept := make([]string, 0, len(roots))
 
@@ -894,19 +912,13 @@ func sendEvent(ctx context.Context, events chan<- walkEvent,
 // operandWarning returns the one-line warning for an operand the walk
 // does not start from, naming the path and what it is, or "" for one it
 // does: a regular file, or a directory not named .zfs, that is not
-// inside a directory named .zfs, neither as given nor with its symlinks
-// resolved. Symlinks are never followed, including as operands.
+// inside a directory named .zfs. resolveRoots has already followed the
+// operand's symlinks, so a symlink here is an operand replaced by one
+// after it was resolved, and is not followed.
 func operandWarning(root string, fi fs.FileInfo) string {
 	// The walk never enters a .zfs directory, so it starts from nothing
-	// inside one either, not even through a symlink. An operand whose
-	// symlinks cannot be resolved is checked as given.
-	resolved, err := filepath.EvalSymlinks(root)
-	if err != nil {
-		resolved = root
-	}
-
-	if slices.Contains(strings.Split(filepath.Dir(root), "/"), ".zfs") ||
-		slices.Contains(strings.Split(filepath.Dir(resolved), "/"), ".zfs") {
+	// inside one either.
+	if slices.Contains(strings.Split(filepath.Dir(root), "/"), ".zfs") {
 		return fmt.Sprintf("walk %s: skipping operand inside a .zfs directory",
 			root)
 	}