Change os.Error convention:

echo back context of call in error if likely to be useful.

For example, if os.Open("/etc/passwd", os.O_RDONLY)
fails with syscall.EPERM, it returns as the os.Error

	&PathError{
		Op: "open",
		Path: "/etc/passwd"
		Error: os.EPERM
	}

which formats as

	open /etc/passwd: permission denied

Not converted:

	datafmt
	go/...
	google/...
	regexp
	tabwriter
	template

R=r
DELTA=1153  (561 added, 156 deleted, 436 changed)
OCL=30738
CL=30781

12 files changed, 196 insertions, 84 deletions

diff --git a/src/pkg/os/dir_darwin_386.go b/src/pkg/os/dir_darwin_386.go
index 2803ecee2..791499d8f 100644
--- a/src/pkg/os/dir_darwin_386.go
+++ b/src/pkg/os/dir_darwin_386.go
@@ -38,9 +38,9 @@ func readdirnames(file *File, count int) (names []string, err Error) {
 			d.nbuf, errno = syscall.Getdirentries(file.fd, d.buf, new(uintptr));
 			if errno != 0 {
 				d.nbuf = 0;
-				return names, ErrnoToError(errno)
+				return names, NewSyscallError("getdirentries", errno);
 			}
-			if d.nbuf == 0 {
+			if d.nbuf <= 0 {
 				break	// EOF
 			}
 		}
diff --git a/src/pkg/os/dir_darwin_amd64.go b/src/pkg/os/dir_darwin_amd64.go
index 2803ecee2..791499d8f 100644
--- a/src/pkg/os/dir_darwin_amd64.go
+++ b/src/pkg/os/dir_darwin_amd64.go
@@ -38,9 +38,9 @@ func readdirnames(file *File, count int) (names []string, err Error) {
 			d.nbuf, errno = syscall.Getdirentries(file.fd, d.buf, new(uintptr));
 			if errno != 0 {
 				d.nbuf = 0;
-				return names, ErrnoToError(errno)
+				return names, NewSyscallError("getdirentries", errno);
 			}
-			if d.nbuf == 0 {
+			if d.nbuf <= 0 {
 				break	// EOF
 			}
 		}
diff --git a/src/pkg/os/dir_linux_386.go b/src/pkg/os/dir_linux_386.go
index c4594a52d..d6d700b24 100644
--- a/src/pkg/os/dir_linux_386.go
+++ b/src/pkg/os/dir_linux_386.go
@@ -47,10 +47,10 @@ func readdirnames(file *File, count int) (names []string, err Error) {
 		if d.bufp >= d.nbuf {
 			var errno int;
 			d.nbuf, errno = syscall.Getdents(file.fd, d.buf);
-			if d.nbuf < 0 {
-				return names, ErrnoToError(errno)
+			if errno != 0 {
+				return names, NewSyscallError("getdents", errno)
 			}
-			if d.nbuf == 0 {
+			if d.nbuf <= 0 {
 				break	// EOF
 			}
 			d.bufp = 0;
diff --git a/src/pkg/os/dir_linux_amd64.go b/src/pkg/os/dir_linux_amd64.go
index 05b3d4c65..8b1664f77 100644
--- a/src/pkg/os/dir_linux_amd64.go
+++ b/src/pkg/os/dir_linux_amd64.go
@@ -43,10 +43,10 @@ func readdirnames(file *File, count int) (names []string, err Error) {
 		if d.bufp >= d.nbuf {
 			var errno int;
 			d.nbuf, errno = syscall.Getdents(file.fd, d.buf);
-			if d.nbuf < 0 {
-				return names, ErrnoToError(errno)
+			if errno != 0 {
+				return names, NewSyscallError("getdents", errno)
 			}
-			if d.nbuf == 0 {
+			if d.nbuf <= 0 {
 				break	// EOF
 			}
 			d.bufp = 0;
diff --git a/src/pkg/os/error.go b/src/pkg/os/error.go
index 718499b21..10a7d042a 100644
--- a/src/pkg/os/error.go
+++ b/src/pkg/os/error.go
@@ -30,15 +30,6 @@ func (e Errno) String() string {
 	return syscall.Errstr(int(e))
 }
 
-// ErrnoToError converts errno to an Error (underneath, an Errno).
-// It returns nil for the "no error" errno.
-func ErrnoToError(errno int) Error {
-	if errno == 0 {
-		return nil
-	}
-	return Errno(errno)
-}
-
 // Commonly known Unix errors.
 var (
 	EPERM Error = Errno(syscall.EPERM);
@@ -81,3 +72,33 @@ var (
 	ENAMETOOLONG Error = Errno(syscall.ENAMETOOLONG);
 )
 
+// PathError records an error and the operation and file path that caused it.
+type PathError struct {
+	Op string;
+	Path string;
+	Error Error;
+}
+
+func (e *PathError) String() string {
+	return e.Op + " " + e.Path + ": " + e.Error.String();
+}
+
+// SyscallError records an error from a specific system call.
+type SyscallError struct {
+	Syscall string;
+	Errno Errno;
+}
+
+func (e *SyscallError) String() string {
+	return e.Syscall + ": " + e.Errno.String();
+}
+
+// NewSyscallError returns, as an os.Error, a new SyscallError
+// with the given system call name and error number.
+// As a convenience, if errno is 0, NewSyscallError returns nil.
+func NewSyscallError(syscall string, errno int) os.Error {
+	if errno == 0 {
+		return nil;
+	}
+	return &SyscallError{syscall, Errno(errno)}
+}
diff --git a/src/pkg/os/exec.go b/src/pkg/os/exec.go
index c1551f86d..a7430ef7f 100644
--- a/src/pkg/os/exec.go
+++ b/src/pkg/os/exec.go
@@ -30,7 +30,10 @@ func ForkExec(argv0 string, argv []string, envv []string, dir string, fd []*File
 	}
 
 	p, e := syscall.ForkExec(argv0, argv, envv, dir, intfd);
-	return int(p), ErrnoToError(e);
+	if e != 0 {
+		return 0, &PathError{"fork/exec", argv0, Errno(e)};
+	}
+	return p, nil;
 }
 
 // Exec replaces the current process with an execution of the program
@@ -42,7 +45,10 @@ func Exec(argv0 string, argv []string, envv []string) Error {
 		envv = Environ();
 	}
 	e := syscall.Exec(argv0, argv, envv);
-	return ErrnoToError(e);
+	if e != 0 {
+		return &PathError{"exec", argv0, Errno(e)};
+	}
+	return nil;
 }
 
 // TODO(rsc): Should os implement its own syscall.WaitStatus
@@ -79,10 +85,10 @@ func Wait(pid int, options int) (w *Waitmsg, err Error) {
 	}
 	pid1, e := syscall.Wait4(pid, &status, options, rusage);
 	if e != 0 {
-		return nil, ErrnoToError(e);
+		return nil, NewSyscallError("wait", e);
 	}
 	w = new(Waitmsg);
-	w.Pid = pid;
+	w.Pid = pid1;
 	w.WaitStatus = status;
 	w.Rusage = rusage;
 	return w, nil;
diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 5b6115932..8c76735fb 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -37,11 +37,11 @@ func (file *File) Name() string {
 }
 
 // NewFile returns a new File with the given file descriptor and name.
-func NewFile(file int, name string) *File {
-	if file < 0 {
+func NewFile(fd int, name string) *File {
+	if fd < 0 {
 		return nil
 	}
-	return &File{file, name, nil, 0}
+	return &File{fd, name, nil, 0}
 }
 
 // Stdin, Stdout, and Stderr are open Files pointing to the standard input,
@@ -74,7 +74,7 @@ const (
 func Open(name string, flag int, perm int) (file *File, err Error) {
 	r, e := syscall.Open(name, flag | syscall.O_CLOEXEC, perm);
 	if e != 0 {
-		return nil, ErrnoToError(e);
+		return nil, &PathError{"open", name, Errno(e)};
 	}
 
 	// There's a race here with fork/exec, which we are
@@ -83,7 +83,7 @@ func Open(name string, flag int, perm int) (file *File, err Error) {
 		syscall.CloseOnExec(r);
 	}
 
-	return NewFile(r, name), ErrnoToError(e)
+	return NewFile(r, name), nil;
 }
 
 // Close closes the File, rendering it unusable for I/O.
@@ -92,7 +92,10 @@ func (file *File) Close() Error {
 	if file == nil {
 		return EINVAL
 	}
-	err := ErrnoToError(syscall.Close(file.fd));
+	var err os.Error;
+	if e := syscall.Close(file.fd); e != 0 {
+		err = &PathError{"close", file.name, Errno(e)};
+	}
 	file.fd = -1;  // so it can't be closed again
 	return err;
 }
@@ -124,7 +127,10 @@ func (file *File) Read(b []byte) (ret int, err Error) {
 	if n == 0 && e == 0 {
 		return 0, EOF
 	}
-	return n, ErrnoToError(e);
+	if e != 0 {
+		err = &PathError{"read", file.name, Errno(e)};
+	}
+	return n, err
 }
 
 // Write writes len(b) bytes to the File.
@@ -146,7 +152,10 @@ func (file *File) Write(b []byte) (ret int, err Error) {
 	} else {
 		file.nepipe = 0;
 	}
-	return n, ErrnoToError(e)
+	if e != 0 {
+		err = &PathError{"write", file.name, Errno(e)};
+	}
+	return n, err
 }
 
 // Seek sets the offset for the next Read or Write on file to offset, interpreted
@@ -155,11 +164,11 @@ func (file *File) Write(b []byte) (ret int, err Error) {
 // It returns the new offset and an Error, if any.
 func (file *File) Seek(offset int64, whence int) (ret int64, err Error) {
 	r, e := syscall.Seek(file.fd, offset, whence);
-	if e != 0 {
-		return -1, ErrnoToError(e)
+	if e == 0 && file.dirinfo != nil && r != 0 {
+		e = syscall.EISDIR;
 	}
-	if file.dirinfo != nil && r != 0 {
-		return -1, ErrnoToError(syscall.EISDIR)
+	if e != 0 {
+		return 0, &PathError{"seek", file.name, Errno(e)};
 	}
 	return r, nil
 }
@@ -172,11 +181,7 @@ func (file *File) WriteString(s string) (ret int, err Error) {
 	}
 	b := syscall.StringByteSlice(s);
 	b = b[0:len(b)-1];
-	r, e := syscall.Write(file.fd, b);
-	if r < 0 {
-		r = 0
-	}
-	return int(r), ErrnoToError(e)
+	return file.Write(b);
 }
 
 // Pipe returns a connected pair of Files; reads from r return bytes written to w.
@@ -189,7 +194,7 @@ func Pipe() (r *File, w *File, err Error) {
 	e := syscall.Pipe(&p);
 	if e != 0 {
 		syscall.ForkLock.RUnlock();
-		return nil, nil, ErrnoToError(e)
+		return nil, nil, NewSyscallError("pipe", e);
 	}
 	syscall.CloseOnExec(p[0]);
 	syscall.CloseOnExec(p[1]);
@@ -201,7 +206,11 @@ func Pipe() (r *File, w *File, err Error) {
 // Mkdir creates a new directory with the specified name and permission bits.
 // It returns an error, if any.
 func Mkdir(name string, perm int) Error {
-	return ErrnoToError(syscall.Mkdir(name, perm));
+	e := syscall.Mkdir(name, perm);
+	if e != 0 {
+		return &PathError{"mkdir", name, Errno(e)};
+	}
+	return nil;
 }
 
 // Stat returns a Dir structure describing the named file and an error, if any.
@@ -213,7 +222,7 @@ func Stat(name string) (dir *Dir, err Error) {
 	var lstat, stat syscall.Stat_t;
 	e := syscall.Lstat(name, &lstat);
 	if e != 0 {
-		return nil, ErrnoToError(e);
+		return nil, &PathError{"stat", name, Errno(e)};
 	}
 	statp := &lstat;
 	if lstat.Mode & syscall.S_IFMT == syscall.S_IFLNK {
@@ -231,7 +240,7 @@ func (file *File) Stat() (dir *Dir, err Error) {
 	var stat syscall.Stat_t;
 	e := syscall.Fstat(file.fd, &stat);
 	if e != 0 {
-		return nil, ErrnoToError(e)
+		return nil, &PathError{"stat", file.name, Errno(e)};
 	}
 	return dirFromStat(file.name, new(Dir), &stat, &stat), nil
 }
@@ -243,7 +252,7 @@ func Lstat(name string) (dir *Dir, err Error) {
 	var stat syscall.Stat_t;
 	e := syscall.Lstat(name, &stat);
 	if e != 0 {
-		return nil, ErrnoToError(e)
+		return nil, &PathError{"lstat", name, Errno(e)};
 	}
 	return dirFromStat(name, new(Dir), &stat, &stat), nil
 }
@@ -290,13 +299,19 @@ func (file *File) Readdir(count int) (dirs []Dir, err Error) {
 
 // Chdir changes the current working directory to the named directory.
 func Chdir(dir string) Error {
-	return ErrnoToError(syscall.Chdir(dir));
+	if e := syscall.Chdir(dir); e != 0 {
+		return &PathError{"chdir", dir, Errno(e)};
+	}
+	return nil;
 }
 
 // Chdir changes the current working directory to the file,
 // which must be a directory.
 func (f *File) Chdir() Error {
-	return ErrnoToError(syscall.Fchdir(f.fd));
+	if e := syscall.Fchdir(f.fd); e != 0 {
+		return &PathError{"chdir", f.name, Errno(e)};
+	}
+	return nil;
 }
 
 // Remove removes the named file or directory.
@@ -326,17 +341,38 @@ func Remove(name string) Error {
 	if e1 != syscall.ENOTDIR {
 		e = e1;
 	}
-	return ErrnoToError(e);
+	return &PathError{"remove", name, Errno(e)};
+}
+
+// LinkError records an error during a link or symlink
+// system call and the paths that caused it.
+type LinkError struct {
+	Op string;
+	Old string;
+	New string;
+	Error Error;
+}
+
+func (e *LinkError) String() string {
+	return e.Op + " " + e.Old + " " + e.New + ": " + e.Error.String();
 }
 
 // Link creates a hard link.
 func Link(oldname, newname string) Error {
-	return ErrnoToError(syscall.Link(oldname, newname));
+	e := syscall.Link(oldname, newname);
+	if e != 0 {
+		return &LinkError{"link", oldname, newname, Errno(e)};
+	}
+	return nil;
 }
 
 // Symlink creates a symbolic link.
 func Symlink(oldname, newname string) Error {
-	return ErrnoToError(syscall.Symlink(oldname, newname));
+	e := syscall.Symlink(oldname, newname);
+	if e != 0 {
+		return &LinkError{"symlink", oldname, newname, Errno(e)};
+	}
+	return nil;
 }
 
 // Readlink reads the contents of a symbolic link: the destination of
@@ -346,7 +382,7 @@ func Readlink(name string) (string, Error) {
 		b := make([]byte, len);
 		n, e := syscall.Readlink(name, b);
 		if e != 0 {
-			return "", ErrnoToError(e);
+			return "", &PathError{"readlink", name, Errno(e)};
 		}
 		if n < len {
 			return string(b[0:n]), nil;
@@ -359,40 +395,61 @@ func Readlink(name string) (string, Error) {
 // Chmod changes the mode of the named file to mode.
 // If the file is a symbolic link, it changes the uid and gid of the link's target.
 func Chmod(name string, mode int) Error {
-	return ErrnoToError(syscall.Chmod(name, mode));
+	if e := syscall.Chmod(name, mode); e != 0 {
+		return &PathError{"chmod", name, Errno(e)};
+	}
+	return nil;
 }
 
 // Chmod changes the mode of the file to mode.
 func (f *File) Chmod(mode int) Error {
-	return ErrnoToError(syscall.Fchmod(f.fd, mode));
+	if e := syscall.Fchmod(f.fd, mode); e != 0 {
+		return &PathError{"chmod", f.name, Errno(e)};
+	}
+	return nil;
 }
 
 // Chown changes the numeric uid and gid of the named file.
 // If the file is a symbolic link, it changes the uid and gid of the link's target.
 func Chown(name string, uid, gid int) Error {
-	return ErrnoToError(syscall.Chown(name, uid, gid));
+	if e := syscall.Chown(name, uid, gid); e != 0 {
+		return &PathError{"chown", name, Errno(e)};
+	}
+	return nil;
 }
 
 // Lchown changes the numeric uid and gid of the named file.
 // If the file is a symbolic link, it changes the uid and gid of the link itself.
 func Lchown(name string, uid, gid int) Error {
-	return ErrnoToError(syscall.Lchown(name, uid, gid));
+	if e := syscall.Lchown(name, uid, gid); e != 0 {
+		return &PathError{"lchown", name, Errno(e)};
+	}
+	return nil;
 }
 
 // Chown changes the numeric uid and gid of the named file.
 func (f *File) Chown(uid, gid int) Error {
-	return ErrnoToError(syscall.Fchown(f.fd, uid, gid));
+	if e := syscall.Fchown(f.fd, uid, gid); e != 0 {
+		return &PathError{"chown", f.name, Errno(e)};
+	}
+	return nil;
 }
 
 // Truncate changes the size of the named file.
 // If the file is a symbolic link, it changes the size of the link's target.
 func Truncate(name string, size int64) Error {
-	return ErrnoToError(syscall.Truncate(name, size));
+	if e := syscall.Truncate(name, size); e != 0 {
+		return &PathError{"truncate", name, Errno(e)};
+	}
+	return nil;
 }
 
 // Truncate changes the size of the file.
 // It does not change the I/O offset.
 func (f *File) Truncate(size int64) Error {
-	return ErrnoToError(syscall.Ftruncate(f.fd, size));
+	if e := syscall.Ftruncate(f.fd, size); e != 0 {
+		return &PathError{"truncate", f.name, Errno(e)};
+	}
+	return nil;
 }
 
diff --git a/src/pkg/os/getwd.go b/src/pkg/os/getwd.go
index 2d7b754b5..cbc6134a7 100644
--- a/src/pkg/os/getwd.go
+++ b/src/pkg/os/getwd.go
@@ -17,7 +17,7 @@ func Getwd() (string, Error) {
 	// If the operating system provides a Getwd call, use it.
 	if syscall.ImplementsGetwd {
 		s, e := syscall.Getwd();
-		return s, ErrnoToError(e);
+		return s, NewSyscallError("getwd", e);
 	}
 
 	// Otherwise, we're trying to find our way back to ".".
diff --git a/src/pkg/os/os_test.go b/src/pkg/os/os_test.go
index 77b69447d..fb555eb86 100644
--- a/src/pkg/os/os_test.go
+++ b/src/pkg/os/os_test.go
@@ -549,3 +549,41 @@ func TestSeek(t *testing.T) {
 	}
 	f.Close();
 }
+
+type openErrorTest struct {
+	path string;
+	mode int;
+	error string;
+}
+
+var openErrorTests = []openErrorTest {
+	openErrorTest {
+		"/etc/no-such-file",
+		O_RDONLY,
+		"open /etc/no-such-file: no such file or directory",
+	},
+	openErrorTest {
+		"/etc",
+		O_WRONLY,
+		"open /etc: is a directory",
+	},
+	openErrorTest {
+		"/etc/passwd/group",
+		O_WRONLY,
+		"open /etc/passwd/group: not a directory",
+	},
+}
+
+func TestOpenError(t *testing.T) {
+	for i, tt := range openErrorTests {
+		f, err := Open(tt.path, tt.mode, 0);
+		if err == nil {
+			t.Errorf("Open(%q, %d) succeeded", tt.path, tt.mode);
+			f.Close();
+			continue;
+		}
+		if s := err.String(); s != tt.error {
+			t.Errorf("Open(%q, %d) = _, %q; want %q", tt.path, tt.mode, s, tt.error);
+		}
+	}
+}
diff --git a/src/pkg/os/path.go b/src/pkg/os/path.go
index 0b86b8f8b..d8efe5183 100644
--- a/src/pkg/os/path.go
+++ b/src/pkg/os/path.go
@@ -6,16 +6,6 @@ package os
 
 import "os"
 
-// PathError reports an error and the file path where it occurred.
-type PathError struct {
-	Path string;
-	Error Error;
-}
-
-func (p *PathError) String() string {
-	return p.Path + ": " + p.Error.String();
-}
-
 // MkdirAll creates a directory named path,
 // along with any necessary parents, and returns nil,
 // or else returns an error.
@@ -30,7 +20,7 @@ func MkdirAll(path string, perm int) Error {
 		if dir.IsDirectory() {
 			return nil;
 		}
-		return &PathError{path, ENOTDIR};
+		return &PathError{"mkdir", path, ENOTDIR};
 	}
 
 	// Doesn't already exist; make sure parent does.
@@ -61,7 +51,7 @@ func MkdirAll(path string, perm int) Error {
 		if err1 == nil && dir.IsDirectory() {
 			return nil;
 		}
-		return &PathError{path, err};
+		return err;
 	}
 	return nil;
 }
@@ -77,19 +67,19 @@ func RemoveAll(path string) Error {
 	}
 
 	// Otherwise, is this a directory we need to recurse into?
-	dir, err1 := os.Lstat(path);
-	if err1 != nil {
-		return &PathError{path, err1};
+	dir, err := os.Lstat(path);
+	if err != nil {
+		return err;
 	}
 	if !dir.IsDirectory() {
 		// Not a directory; return the error from Remove.
-		return &PathError{path, err};
+		return err;
 	}
 
 	// Directory.
 	fd, err := Open(path, os.O_RDONLY, 0);
 	if err != nil {
-		return &PathError{path, err};
+		return err;
 	}
 	defer fd.Close();
 
@@ -99,13 +89,13 @@ func RemoveAll(path string) Error {
 		names, err1 := fd.Readdirnames(100);
 		for i, name := range names {
 			err1 := RemoveAll(path + "/" + name);
-			if err1 != nil && err == nil {
+			if err == nil {
 				err = err1;
 			}
 		}
 		// If Readdirnames returned an error, use it.
-		if err1 != nil && err == nil {
-			err = &PathError{path, err1};
+		if err == nil {
+			err = err1;
 		}
 		if len(names) == 0 {
 			break;
@@ -113,9 +103,9 @@ func RemoveAll(path string) Error {
 	}
 
 	// Remove directory.
-	err1 = Remove(path);
-	if err1 != nil && err == nil {
-		err = &PathError{path, err1};
+	err1 := Remove(path);
+	if err == nil {
+		err = err1;
 	}
 	return err;
 }
diff --git a/src/pkg/os/proc.go b/src/pkg/os/proc.go
index d2fd6493e..9920c1355 100644
--- a/src/pkg/os/proc.go
+++ b/src/pkg/os/proc.go
@@ -39,7 +39,7 @@ func Getegid() int {
 // Getgroups returns a list of the numeric ids of groups that the caller belongs to.
 func Getgroups() ([]int, os.Error) {
 	gids, errno := syscall.Getgroups();
-	return gids, ErrnoToError(errno);
+	return gids, NewSyscallError("getgroups", errno);
 }
 
 // Exit causes the current program to exit with the given status code.
diff --git a/src/pkg/os/time.go b/src/pkg/os/time.go
index 3eee243cc..e1022066f 100644
--- a/src/pkg/os/time.go
+++ b/src/pkg/os/time.go
@@ -17,7 +17,7 @@ import (
 func Time() (sec int64, nsec int64, err Error) {
 	var tv syscall.Timeval;
 	if errno := syscall.Gettimeofday(&tv); errno != 0 {
-		return 0, 0, ErrnoToError(errno)
+		return 0, 0, NewSyscallError("gettimeofday", errno);
 	}
 	return int64(tv.Sec), int64(tv.Usec)*1000, err;
 }