cmd/go: reject case-insensitive file name, import collisions

To make sure that Go code will work when moved to a
system with a case-insensitive file system, like OS X or Windows,
reject any package built from files with names differing
only in case, and also any package built from imported
dependencies with names differing only in case.

Fixes #4773.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7314104

src/cmd/go/main.go

@@ -589,3 +589,60 @@ func stringList(args ...interface{}) []string {
 	}
 	return x
 }
+
+// toFold returns a string with the property that
+//	strings.EqualFold(s, t) iff toFold(s) == toFold(t)
+// This lets us test a large set of strings for fold-equivalent
+// duplicates without making a quadratic number of calls
+// to EqualFold. Note that strings.ToUpper and strings.ToLower
+// have the desired property in some corner cases.
+func toFold(s string) string {
+	// Fast path: all ASCII, no upper case.
+	// Most paths look like this already.
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf || 'A' <= c && c <= 'Z' {
+			goto Slow
+		}
+	}
+	return s
+
+Slow:
+	var buf bytes.Buffer
+	for _, r := range s {
+		// SimpleFold(x) cycles to the next equivalent rune > x
+		// or wraps around to smaller values. Iterate until it wraps,
+		// and we've found the minimum value.
+		for {
+			r0 := r
+			r = unicode.SimpleFold(r0)
+			if r <= r0 {
+				break
+			}
+		}
+		// Exception to allow fast path above: A-Z => a-z
+		if 'A' <= r && r <= 'Z' {
+			r += 'a' - 'A'
+		}
+		buf.WriteRune(r)
+	}
+	return buf.String()
+}
+
+// foldDup reports a pair of strings from the list that are
+// equal according to strings.EqualFold.
+// It returns "", "" if there are no such strings.
+func foldDup(list []string) (string, string) {
+	clash := map[string]string{}
+	for _, s := range list {
+		fold := toFold(s)
+		if t := clash[fold]; t != "" {
+			if s > t {
+				s, t = t, s
+			}
+			return s, t
+		}
+		clash[fold] = s
+	}
+	return "", ""
+}

src/cmd/go/pkg.go

@@ -125,6 +125,9 @@ func (p *PackageError) Error() string {
 		// is the most important thing.
 		return p.Pos + ": " + p.Err
 	}
+	if len(p.ImportStack) == 0 {
+		return p.Err
+	}
 	return "package " + strings.Join(p.ImportStack, "\n\timports ") + ": " + p.Err
 }
 
@@ -370,6 +373,31 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	p.allgofiles = append(p.allgofiles, p.gofiles...)
 	sort.Strings(p.allgofiles)
 
+	// Check for case-insensitive collision of input files.
+	// To avoid problems on case-insensitive files, we reject any package
+	// where two different input files have equal names under a case-insensitive
+	// comparison.
+	f1, f2 := foldDup(stringList(
+		p.GoFiles,
+		p.CgoFiles,
+		p.IgnoredGoFiles,
+		p.CFiles,
+		p.HFiles,
+		p.SFiles,
+		p.SysoFiles,
+		p.SwigFiles,
+		p.SwigCXXFiles,
+		p.TestGoFiles,
+		p.XTestGoFiles,
+	))
+	if f1 != "" {
+		p.Error = &PackageError{
+			ImportStack: stk.copy(),
+			Err:         fmt.Sprintf("case-insensitive file name collision: %q and %q", f1, f2),
+		}
+		return p
+	}
+
 	// Build list of imported packages and full dependency list.
 	imports := make([]*Package, 0, len(p.Imports))
 	deps := make(map[string]bool)
@@ -423,8 +451,21 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	if p.Standard && (p.ImportPath == "unsafe" || buildContext.Compiler == "gccgo") {
 		p.target = ""
 	}
-
 	p.Target = p.target
+
+	// In the absence of errors lower in the dependency tree,
+	// check for case-insensitive collisions of import paths.
+	if len(p.DepsErrors) == 0 {
+		dep1, dep2 := foldDup(p.Deps)
+		if dep1 != "" {
+			p.Error = &PackageError{
+				ImportStack: stk.copy(),
+				Err:         fmt.Sprintf("case-insensitive import collision: %q and %q", dep1, dep2),
+			}
+			return p
+		}
+	}
+
 	return p
 }
 

src/cmd/go/test.bash

@@ -279,6 +279,47 @@ fi
 unset GOPATH
 rm -rf $d
 
+# issue 4773. case-insensitive collisions
+d=$(mktemp -d -t testgo)
+export GOPATH=$d
+mkdir -p $d/src/example/a $d/src/example/b
+cat >$d/src/example/a/a.go <<EOF
+package p
+import (
+	_ "math/rand"
+	_ "math/Rand"
+)
+EOF
+if ./testgo list example/a 2>$d/out; then
+	echo go list example/a should have failed, did not.
+	ok=false
+elif ! grep "case-insensitive import collision" $d/out >/dev/null; then
+	echo go list example/a did not report import collision.
+	ok=false
+fi
+cat >$d/src/example/b/file.go <<EOF
+package b
+EOF
+cat >$d/src/example/b/FILE.go <<EOF
+package b
+EOF
+if [ $(ls $d/src/example/b | wc -l) = 2 ]; then
+	# case-sensitive file system, let directory read find both files
+	args="example/b"
+else
+	# case-insensitive file system, list files explicitly on command line.
+	args="$d/src/example/b/file.go $d/src/example/b/FILE.go"
+fi
+if ./testgo list $args 2>$d/out; then
+	echo go list example/b should have failed, did not.
+	ok=false
+elif ! grep "case-insensitive file name collision" $d/out >/dev/null; then
+	echo go list example/b did not report file name collision.
+	ok=false
+fi
+unset GOPATH
+rm -rf $d
+
 # Only succeeds if source order is preserved.
 ./testgo test testdata/example[12]_test.go