cmd/go/internal/fsys: refactor overlay consistency checks

Do the overlay consistency checks separate from constructing
the overlay data structure. This makes sure that the data structure
can be changed without worrying about losing the checks.

Change-Id: I9ff50cc366b5362adc5570f94e6caf646ddf5046
Reviewed-on: https://go-review.googlesource.com/c/go/+/628700
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/fsys/fsys.go

@@ -88,6 +88,11 @@ type overlayJSON struct {
 	Replace map[string]string
 }
 
+type replace struct {
+	from string
+	to   string
+}
+
 type node struct {
 	actual   string           // empty if a directory
 	children map[string]*node // path element → file or directory
@@ -230,78 +235,69 @@ func initFromJSON(js []byte) error {
 		return fmt.Errorf("parsing overlay JSON: %v", err)
 	}
 
-	// Canonicalize the paths in the overlay map.
-	// Use reverseCanonicalized to check for collisions:
-	// no two 'from' paths should abs to the same path.
-	overlay = make(map[string]*node)
-	reverseCanonicalized := make(map[string]string) // inverse of abs operation, to check for duplicates
-	// Build a table of file and directory nodes from the replacement map.
-
+	seen := make(map[string]string)
+	var list []replace
 	for _, from := range slices.Sorted(maps.Keys(ojs.Replace)) {
-		to := ojs.Replace[from]
-		// Canonicalize paths and check for a collision.
 		if from == "" {
 			return fmt.Errorf("empty string key in overlay map")
 		}
-		cfrom := abs(from)
-		to = abs(to)
-		if otherFrom, seen := reverseCanonicalized[cfrom]; seen {
-			return fmt.Errorf(
-				"duplicate paths %s and %s in overlay map", otherFrom, from)
+		afrom := abs(from)
+		if old, ok := seen[afrom]; ok {
+			return fmt.Errorf("duplicate paths %s and %s in overlay map", old, from)
 		}
-		reverseCanonicalized[cfrom] = from
-		from = cfrom
+		seen[afrom] = from
+		list = append(list, replace{from: afrom, to: ojs.Replace[from]})
+	}
 
-		// Create node for overlaid file.
-		dir, base := filepath.Dir(from), filepath.Base(from)
-		if n, ok := overlay[from]; ok {
-			// All 'from' paths in the overlay are file paths. Since the from paths
-			// are in a map, they are unique, so if the node already exists we added
-			// it below when we create parent directory nodes. That is, that
-			// both a file and a path to one of its parent directories exist as keys
-			// in the Replace map.
-			//
-			// This only applies if the overlay directory has any files or directories
-			// in it: placeholder directories that only contain deleted files don't
-			// count. They are safe to be overwritten with actual files.
-			for fname, f := range n.children {
-				if !f.isDeleted() {
-					return fmt.Errorf("inconsistent files %s and %s in overlay map", filepath.Join(from, fname), from)
-				}
-			}
+	slices.SortFunc(list, func(x, y replace) int { return cmp(x.from, y.from) })
+
+	for i, r := range list {
+		if r.to == "" { // deleted
+			continue
 		}
-		overlay[from] = &node{actual: to}
-
-		// Add parent directory nodes to overlay structure.
-		childNode := overlay[from]
-		for {
-			dirNode := overlay[dir]
-			if dirNode == nil || dirNode.isDeleted() {
-				dirNode = &node{children: make(map[string]*node)}
-				overlay[dir] = dirNode
-			}
-			if childNode.isDeleted() {
-				// Only create one parent for a deleted file:
-				// the directory only conditionally exists if
-				// there are any non-deleted children, so
-				// we don't create their parents.
-				if dirNode.isDir() {
-					dirNode.children[base] = childNode
-				}
+		// have file for r.from; look for child file implying r.from is a directory
+		prefix := r.from + string(filepath.Separator)
+		for _, next := range list[i+1:] {
+			if !strings.HasPrefix(next.from, prefix) {
 				break
 			}
-			if !dirNode.isDir() {
-				// This path already exists as a file, so it can't be a parent
-				// directory. See comment at error above.
-				return fmt.Errorf("inconsistent files %s and %s in overlay map", dir, from)
+			if next.to != "" {
+				// found child file
+				return fmt.Errorf("inconsistent files %s and %s in overlay map", r.from, next.from)
 			}
-			dirNode.children[base] = childNode
-			parent := filepath.Dir(dir)
-			if parent == dir {
-				break // reached the top; there is no parent
+		}
+	}
+
+	overlay = make(map[string]*node)
+	for _, r := range list {
+		n := &node{actual: abs(r.to)}
+		from := r.from
+		overlay[from] = n
+
+		for {
+			dir, base := filepath.Dir(from), filepath.Base(from)
+			if dir == from {
+				break
 			}
-			dir, base = parent, filepath.Base(dir)
-			childNode = dirNode
+			dn := overlay[dir]
+			if dn == nil || dn.isDeleted() {
+				dn = &node{children: make(map[string]*node)}
+				overlay[dir] = dn
+			}
+			if n.isDeleted() && !dn.isDir() {
+				break
+			}
+			if !dn.isDir() {
+				panic("fsys inconsistency")
+			}
+			dn.children[base] = n
+			if n.isDeleted() {
+				// Deletion is recorded now.
+				// Don't need to create entire parent chain,
+				// because we don't need to force parents to exist.
+				break
+			}
+			from, n = dir, dn
 		}
 	}
 
@@ -591,3 +587,20 @@ func (f fakeDir) Sys() any           { return nil }
 func (f fakeDir) String() string {
 	return fs.FormatFileInfo(f)
 }
+
+func cmp(x, y string) int {
+	for i := 0; i < len(x) && i < len(y); i++ {
+		xi := int(x[i])
+		yi := int(y[i])
+		if xi == filepath.Separator {
+			xi = -1
+		}
+		if yi == filepath.Separator {
+			yi = -1
+		}
+		if xi != yi {
+			return xi - yi
+		}
+	}
+	return len(x) - len(y)
+}

src/cmd/go/internal/fsys/fsys_test.go

@@ -1146,12 +1146,10 @@ var badOverlayTests = []struct {
 	{`{"Replace": {"/tmp/x": "y", "x": "y"}}`,
 		`duplicate paths /tmp/x and x in overlay map`},
 	{`{"Replace": {"/tmp/x/z": "z", "x":"y"}}`,
-		`inconsistent files /tmp/x/z and /tmp/x in overlay map`},
+		`inconsistent files /tmp/x and /tmp/x/z in overlay map`},
 	{`{"Replace": {"/tmp/x/z/z2": "z", "x":"y"}}`,
-		// TODO: Error should say /tmp/x/z/z2
-		`inconsistent files /tmp/x/z and /tmp/x in overlay map`},
+		`inconsistent files /tmp/x and /tmp/x/z/z2 in overlay map`},
 	{`{"Replace": {"/tmp/x": "y", "x/z/z2": "z"}}`,
-		// TODO: Error should say /tmp/x/z/z2
 		`inconsistent files /tmp/x and /tmp/x/z/z2 in overlay map`},
 }