Simplify FD handling for container start/exec

VFS1 and VFS2 host FDs have different dupping behavior,
making error prone to code for both. Change the contract
so that FDs are released as they are used, so the caller
can simple defer a block that closes all remaining files.
This also addresses handling of partial failures.

With this fix, more VFS2 tests can be enabled.

Updates #1487

PiperOrigin-RevId: 330112266

1 files changed, 32 insertions, 10 deletions

diff --git a/pkg/fd/fd.go b/pkg/fd/fd.go
index 83bcfe220..cc6b0cdf1 100644
--- a/pkg/fd/fd.go
+++ b/pkg/fd/fd.go
@@ -49,7 +49,7 @@ func fixCount(n int, err error) (int, error) {
 
 // Read implements io.Reader.
 func (r *ReadWriter) Read(b []byte) (int, error) {
-	c, err := fixCount(syscall.Read(int(atomic.LoadInt64(&r.fd)), b))
+	c, err := fixCount(syscall.Read(r.FD(), b))
 	if c == 0 && len(b) > 0 && err == nil {
 		return 0, io.EOF
 	}
@@ -62,7 +62,7 @@ func (r *ReadWriter) Read(b []byte) (int, error) {
 func (r *ReadWriter) ReadAt(b []byte, off int64) (c int, err error) {
 	for len(b) > 0 {
 		var m int
-		m, err = fixCount(syscall.Pread(int(atomic.LoadInt64(&r.fd)), b, off))
+		m, err = fixCount(syscall.Pread(r.FD(), b, off))
 		if m == 0 && err == nil {
 			return c, io.EOF
 		}
@@ -82,7 +82,7 @@ func (r *ReadWriter) Write(b []byte) (int, error) {
 	var n, remaining int
 	for remaining = len(b); remaining > 0; {
 		woff := len(b) - remaining
-		n, err = syscall.Write(int(atomic.LoadInt64(&r.fd)), b[woff:])
+		n, err = syscall.Write(r.FD(), b[woff:])
 
 		if n > 0 {
 			// syscall.Write wrote some bytes. This is the common case.
@@ -110,7 +110,7 @@ func (r *ReadWriter) Write(b []byte) (int, error) {
 func (r *ReadWriter) WriteAt(b []byte, off int64) (c int, err error) {
 	for len(b) > 0 {
 		var m int
-		m, err = fixCount(syscall.Pwrite(int(atomic.LoadInt64(&r.fd)), b, off))
+		m, err = fixCount(syscall.Pwrite(r.FD(), b, off))
 		if err != nil {
 			break
 		}
@@ -121,6 +121,16 @@ func (r *ReadWriter) WriteAt(b []byte, off int64) (c int, err error) {
 	return
 }
 
+// FD returns the owned file descriptor. Ownership remains unchanged.
+func (r *ReadWriter) FD() int {
+	return int(atomic.LoadInt64(&r.fd))
+}
+
+// String implements Stringer.String().
+func (r *ReadWriter) String() string {
+	return fmt.Sprintf("FD: %d", r.FD())
+}
+
 // FD owns a host file descriptor.
 //
 // It is similar to os.File, with a few important distinctions:
@@ -167,6 +177,23 @@ func NewFromFile(file *os.File) (*FD, error) {
 	return New(fd), nil
 }
 
+// NewFromFiles creates new FDs for each file in the slice.
+func NewFromFiles(files []*os.File) ([]*FD, error) {
+	rv := make([]*FD, 0, len(files))
+	for _, f := range files {
+		new, err := NewFromFile(f)
+		if err != nil {
+			// Cleanup on error.
+			for _, fd := range rv {
+				fd.Close()
+			}
+			return nil, err
+		}
+		rv = append(rv, new)
+	}
+	return rv, nil
+}
+
 // Open is equivalent to open(2).
 func Open(path string, openmode int, perm uint32) (*FD, error) {
 	f, err := syscall.Open(path, openmode|syscall.O_LARGEFILE, perm)
@@ -204,11 +231,6 @@ func (f *FD) Release() int {
 	return int(atomic.SwapInt64(&f.fd, -1))
 }
 
-// FD returns the file descriptor owned by FD. FD retains ownership.
-func (f *FD) FD() int {
-	return int(atomic.LoadInt64(&f.fd))
-}
-
 // File converts the FD to an os.File.
 //
 // FD does not transfer ownership of the file descriptor (it will be
@@ -219,7 +241,7 @@ func (f *FD) FD() int {
 // This operation is somewhat expensive, so care should be taken to minimize
 // its use.
 func (f *FD) File() (*os.File, error) {
-	fd, err := syscall.Dup(int(atomic.LoadInt64(&f.fd)))
+	fd, err := syscall.Dup(f.FD())
 	if err != nil {
 		return nil, err
 	}