path/filepath: use windows GetShortPathName api to force GetLongPathName to do its work

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5928043

api/go1.txt

@@ -22244,6 +22244,7 @@ pkg syscall (windows-386), func GetProcessTimes(Handle, *Filetime, *Filetime, *F
 pkg syscall (windows-386), func GetProtoByName(string) (*Protoent, error)
 pkg syscall (windows-386), func GetQueuedCompletionStatus(Handle, *uint32, *uint32, **Overlapped, uint32) error
 pkg syscall (windows-386), func GetServByName(string, string) (*Servent, error)
+pkg syscall (windows-386), func GetShortPathName(*uint16, *uint16, uint32) (uint32, error)
 pkg syscall (windows-386), func GetStartupInfo(*StartupInfo) error
 pkg syscall (windows-386), func GetStdHandle(int) (Handle, error)
 pkg syscall (windows-386), func GetSystemTimeAsFileTime(*Filetime)
@@ -23161,6 +23162,7 @@ pkg syscall (windows-amd64), func GetProcessTimes(Handle, *Filetime, *Filetime,
 pkg syscall (windows-amd64), func GetProtoByName(string) (*Protoent, error)
 pkg syscall (windows-amd64), func GetQueuedCompletionStatus(Handle, *uint32, *uint32, **Overlapped, uint32) error
 pkg syscall (windows-amd64), func GetServByName(string, string) (*Servent, error)
+pkg syscall (windows-amd64), func GetShortPathName(*uint16, *uint16, uint32) (uint32, error)
 pkg syscall (windows-amd64), func GetStartupInfo(*StartupInfo) error
 pkg syscall (windows-amd64), func GetStdHandle(int) (Handle, error)
 pkg syscall (windows-amd64), func GetSystemTimeAsFileTime(*Filetime)

src/pkg/path/filepath/symlink_windows.go

@@ -8,7 +8,24 @@ import (
 	"syscall"
 )
 
-func evalSymlinks(path string) (string, error) {
+func toShort(path string) (string, error) {
+	p := syscall.StringToUTF16(path)
+	b := p // GetShortPathName says we can reuse buffer
+	n, err := syscall.GetShortPathName(&p[0], &b[0], uint32(len(b)))
+	if err != nil {
+		return "", err
+	}
+	if n > uint32(len(b)) {
+		b = make([]uint16, n)
+		n, err = syscall.GetShortPathName(&p[0], &b[0], uint32(len(b)))
+		if err != nil {
+			return "", err
+		}
+	}
+	return syscall.UTF16ToString(b), nil
+}
+
+func toLong(path string) (string, error) {
 	p := syscall.StringToUTF16(path)
 	b := p // GetLongPathName says we can reuse buffer
 	n, err := syscall.GetLongPathName(&p[0], &b[0], uint32(len(b)))
@@ -23,13 +40,24 @@ func evalSymlinks(path string) (string, error) {
 		}
 	}
 	b = b[:n]
-	s := syscall.UTF16ToString(b)
+	return syscall.UTF16ToString(b), nil
+}
+
+func evalSymlinks(path string) (string, error) {
+	p, err := toShort(path)
+	if err != nil {
+		return "", err
+	}
+	p, err = toLong(p)
+	if err != nil {
+		return "", err
+	}
 	// syscall.GetLongPathName does not change the case of the drive letter,
 	// but the result of EvalSymlinks must be unique, so we have
 	// EvalSymlinks(`c:\a`) == EvalSymlinks(`C:\a`).
 	// Make drive letter upper case. This matches what os.Getwd returns.
-	if len(s) >= 2 && s[1] == ':' && 'a' <= s[0] && s[0] <= 'z' {
-		s = string(s[0]+'A'-'a') + s[1:]
+	if len(p) >= 2 && p[1] == ':' && 'a' <= p[0] && p[0] <= 'z' {
+		p = string(p[0]+'A'-'a') + p[1:]
 	}
-	return Clean(s), nil
+	return Clean(p), nil
 }

src/pkg/syscall/syscall_windows.go

@@ -175,6 +175,7 @@ func NewCallback(fn interface{}) uintptr
 //sys	FlushFileBuffers(handle Handle) (err error)
 //sys	GetFullPathName(path *uint16, buflen uint32, buf *uint16, fname **uint16) (n uint32, err error) = kernel32.GetFullPathNameW
 //sys	GetLongPathName(path *uint16, buf *uint16, buflen uint32) (n uint32, err error) = kernel32.GetLongPathNameW
+//sys	GetShortPathName(longpath *uint16, shortpath *uint16, buflen uint32) (n uint32, err error) = kernel32.GetShortPathNameW
 //sys	CreateFileMapping(fhandle Handle, sa *SecurityAttributes, prot uint32, maxSizeHigh uint32, maxSizeLow uint32, name *uint16) (handle Handle, err error) = kernel32.CreateFileMappingW
 //sys	MapViewOfFile(handle Handle, access uint32, offsetHigh uint32, offsetLow uint32, length uintptr) (addr uintptr, err error)
 //sys	UnmapViewOfFile(addr uintptr) (err error)

src/pkg/syscall/zsyscall_windows_386.go

@@ -79,6 +79,7 @@ var (
 	procFlushFileBuffers                 = modkernel32.NewProc("FlushFileBuffers")
 	procGetFullPathNameW                 = modkernel32.NewProc("GetFullPathNameW")
 	procGetLongPathNameW                 = modkernel32.NewProc("GetLongPathNameW")
+	procGetShortPathNameW                = modkernel32.NewProc("GetShortPathNameW")
 	procCreateFileMappingW               = modkernel32.NewProc("CreateFileMappingW")
 	procMapViewOfFile                    = modkernel32.NewProc("MapViewOfFile")
 	procUnmapViewOfFile                  = modkernel32.NewProc("UnmapViewOfFile")
@@ -904,6 +905,19 @@ func GetLongPathName(path *uint16, buf *uint16, buflen uint32) (n uint32, err er
 	return
 }
 
+func GetShortPathName(longpath *uint16, shortpath *uint16, buflen uint32) (n uint32, err error) {
+	r0, _, e1 := Syscall(procGetShortPathNameW.Addr(), 3, uintptr(unsafe.Pointer(longpath)), uintptr(unsafe.Pointer(shortpath)), uintptr(buflen))
+	n = uint32(r0)
+	if n == 0 {
+		if e1 != 0 {
+			err = error(e1)
+		} else {
+			err = EINVAL
+		}
+	}
+	return
+}
+
 func CreateFileMapping(fhandle Handle, sa *SecurityAttributes, prot uint32, maxSizeHigh uint32, maxSizeLow uint32, name *uint16) (handle Handle, err error) {
 	r0, _, e1 := Syscall6(procCreateFileMappingW.Addr(), 6, uintptr(fhandle), uintptr(unsafe.Pointer(sa)), uintptr(prot), uintptr(maxSizeHigh), uintptr(maxSizeLow), uintptr(unsafe.Pointer(name)))
 	handle = Handle(r0)

src/pkg/syscall/zsyscall_windows_amd64.go

@@ -79,6 +79,7 @@ var (
 	procFlushFileBuffers                 = modkernel32.NewProc("FlushFileBuffers")
 	procGetFullPathNameW                 = modkernel32.NewProc("GetFullPathNameW")
 	procGetLongPathNameW                 = modkernel32.NewProc("GetLongPathNameW")
+	procGetShortPathNameW                = modkernel32.NewProc("GetShortPathNameW")
 	procCreateFileMappingW               = modkernel32.NewProc("CreateFileMappingW")
 	procMapViewOfFile                    = modkernel32.NewProc("MapViewOfFile")
 	procUnmapViewOfFile                  = modkernel32.NewProc("UnmapViewOfFile")
@@ -904,6 +905,19 @@ func GetLongPathName(path *uint16, buf *uint16, buflen uint32) (n uint32, err er
 	return
 }
 
+func GetShortPathName(longpath *uint16, shortpath *uint16, buflen uint32) (n uint32, err error) {
+	r0, _, e1 := Syscall(procGetShortPathNameW.Addr(), 3, uintptr(unsafe.Pointer(longpath)), uintptr(unsafe.Pointer(shortpath)), uintptr(buflen))
+	n = uint32(r0)
+	if n == 0 {
+		if e1 != 0 {
+			err = error(e1)
+		} else {
+			err = EINVAL
+		}
+	}
+	return
+}
+
 func CreateFileMapping(fhandle Handle, sa *SecurityAttributes, prot uint32, maxSizeHigh uint32, maxSizeLow uint32, name *uint16) (handle Handle, err error) {
 	r0, _, e1 := Syscall6(procCreateFileMappingW.Addr(), 6, uintptr(fhandle), uintptr(unsafe.Pointer(sa)), uintptr(prot), uintptr(maxSizeHigh), uintptr(maxSizeLow), uintptr(unsafe.Pointer(name)))
 	handle = Handle(r0)