unix: retry fetching of lists through sysctl if the size changes

On macOS, the SysctlKinfoProcSlice() function may be used to fetch the contents of the process table. As the process table may grow between the first and second call to sysctl(), the second call may fail with ENOMEM. In that case we simply need to retry.

Change-Id: I40229653ed383603c33762f37b0dc2e7de47bcb6
GitHub-Last-Rev: b63b6b8
GitHub-Pull-Request: #169
Reviewed-on: https://go-review.googlesource.com/c/sys/+/513036
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/syscall_darwin.go

@@ -510,30 +510,36 @@ func SysctlKinfoProcSlice(name string, args ...int) ([]KinfoProc, error) {
 		return nil, err
 	}
 
-	// Find size.
-	n := uintptr(0)
-	if err := sysctl(mib, nil, &n, nil, 0); err != nil {
-		return nil, err
-	}
-	if n == 0 {
-		return nil, nil
-	}
-	if n%SizeofKinfoProc != 0 {
-		return nil, fmt.Errorf("sysctl() returned a size of %d, which is not a multiple of %d", n, SizeofKinfoProc)
-	}
+	for {
+		// Find size.
+		n := uintptr(0)
+		if err := sysctl(mib, nil, &n, nil, 0); err != nil {
+			return nil, err
+		}
+		if n == 0 {
+			return nil, nil
+		}
+		if n%SizeofKinfoProc != 0 {
+			return nil, fmt.Errorf("sysctl() returned a size of %d, which is not a multiple of %d", n, SizeofKinfoProc)
+		}
 
-	// Read into buffer of that size.
-	buf := make([]KinfoProc, n/SizeofKinfoProc)
-	if err := sysctl(mib, (*byte)(unsafe.Pointer(&buf[0])), &n, nil, 0); err != nil {
-		return nil, err
-	}
-	if n%SizeofKinfoProc != 0 {
-		return nil, fmt.Errorf("sysctl() returned a size of %d, which is not a multiple of %d", n, SizeofKinfoProc)
-	}
+		// Read into buffer of that size.
+		buf := make([]KinfoProc, n/SizeofKinfoProc)
+		if err := sysctl(mib, (*byte)(unsafe.Pointer(&buf[0])), &n, nil, 0); err != nil {
+			if err == ENOMEM {
+				// Process table grew. Try again.
+				continue
+			}
+			return nil, err
+		}
+		if n%SizeofKinfoProc != 0 {
+			return nil, fmt.Errorf("sysctl() returned a size of %d, which is not a multiple of %d", n, SizeofKinfoProc)
+		}
 
-	// The actual call may return less than the original reported required
-	// size so ensure we deal with that.
-	return buf[:n/SizeofKinfoProc], nil
+		// The actual call may return less than the original reported required
+		// size so ensure we deal with that.
+		return buf[:n/SizeofKinfoProc], nil
+	}
 }
 
 //sys	sendfile(infd int, outfd int, offset int64, len *int64, hdtr unsafe.Pointer, flags int) (err error)