internal/poll: move isBlocking checks to inside execIO

This is a step towards deferring adding the handle to IOCP until the
first IO operation.

Having all isBlocking checks inside execIO will make it easier to delay
getting that information until the first IO operation.

It also makes the code simpler, as Pread and Pwrite now only modify
the overlapped object offset, not the one in the FD struct, so they
don't need to reverse the offset change after the IO operation.

For #76391

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest,gotip-windows-amd64-race
Change-Id: Iea680c502c9417b9569a0b1a9da6f7e6bf916f6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/742283
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 31 insertions, 36 deletions

diff --git a/src/internal/poll/fd_windows.go b/src/internal/poll/fd_windows.go
index fa043dfa83..949c5ea938 100644
--- a/src/internal/poll/fd_windows.go
+++ b/src/internal/poll/fd_windows.go
@@ -82,6 +82,11 @@ type operation struct {
 	mode       int32
 }
 
+func (o *operation) setOffset(off int64) {
+	o.o.OffsetHigh = uint32(off >> 32)
+	o.o.Offset = uint32(off)
+}
+
 func (fd *FD) overlapped(o *operation) *syscall.Overlapped {
 	if fd.isBlocking {
 		// Don't return the overlapped object if the file handle
@@ -245,13 +250,10 @@ func (fd *FD) execIO(mode int, submit func(o *operation) (uint32, error), buf []
 	o := operationPool.Get().(*operation)
 	defer operationPool.Put(o)
 	*o = operation{
-		o: syscall.Overlapped{
-			OffsetHigh: uint32(fd.offset >> 32),
-			Offset:     uint32(fd.offset),
-		},
 		runtimeCtx: fd.pd.runtimeCtx,
 		mode:       int32(mode),
 	}
+	o.setOffset(fd.offset)
 	if !fd.isBlocking {
 		if len(buf) > 0 {
 			ptr := unsafe.SliceData(buf)
@@ -396,7 +398,7 @@ func (fd *FD) setOffset(off int64) {
 
 // addOffset adds the given offset to the current offset.
 func (fd *FD) addOffset(off int) {
-	fd.setOffset(fd.offset + int64(off))
+	fd.offset += int64(off)
 }
 
 // pollable should be used instead of fd.pd.pollable(),
@@ -685,24 +687,21 @@ func (fd *FD) Pread(buf []byte, off int64) (int, error) {
 		buf = buf[:maxRW]
 	}
 
-	if fd.isBlocking {
-		curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
-		if err != nil {
-			return 0, err
-		}
-		defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
-		defer fd.setOffset(curoffset)
-	} else {
+	n, err := fd.execIO('r', func(o *operation) (qty uint32, err error) {
 		// Overlapped handles don't have the file pointer updated
 		// when performing I/O operations, so there is no need to
 		// call Seek to reset the file pointer.
 		// Also, some overlapped file handles don't support seeking.
 		// See https://go.dev/issues/74951.
-		curoffset := fd.offset
-		defer fd.setOffset(curoffset)
-	}
-	fd.setOffset(off)
-	n, err := fd.execIO('r', func(o *operation) (qty uint32, err error) {
+		if fd.isBlocking {
+			curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
+			if err != nil {
+				return 0, err
+			}
+			defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
+		}
+		o.setOffset(off)
+
 		err = syscall.ReadFile(fd.Sysfd, buf, &qty, &o.o)
 		return qty, err
 	}, buf)
@@ -908,31 +907,27 @@ func (fd *FD) Pwrite(buf []byte, off int64) (int, error) {
 	}
 	defer fd.readWriteUnlock()
 
-	if fd.isBlocking {
-		curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
-		if err != nil {
-			return 0, err
-		}
-		defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
-		defer fd.setOffset(curoffset)
-	} else {
-		// Overlapped handles don't have the file pointer updated
-		// when performing I/O operations, so there is no need to
-		// call Seek to reset the file pointer.
-		// Also, some overlapped file handles don't support seeking.
-		// See https://go.dev/issues/74951.
-		curoffset := fd.offset
-		defer fd.setOffset(curoffset)
-	}
-
 	var ntotal int
 	for {
 		max := len(buf)
 		if max-ntotal > maxRW {
 			max = ntotal + maxRW
 		}
-		fd.setOffset(off + int64(ntotal))
 		n, err := fd.execIO('w', func(o *operation) (qty uint32, err error) {
+			// Overlapped handles don't have the file pointer updated
+			// when performing I/O operations, so there is no need to
+			// call Seek to reset the file pointer.
+			// Also, some overlapped file handles don't support seeking.
+			// See https://go.dev/issues/74951.
+			if fd.isBlocking {
+				curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
+				if err != nil {
+					return 0, err
+				}
+				defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
+			}
+			o.setOffset(off + int64(ntotal))
+
 			err = syscall.WriteFile(fd.Sysfd, buf[ntotal:max], &qty, &o.o)
 			return qty, err
 		}, buf[ntotal:max])