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.

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. Retry if ENOMEM is
+		// returned, as it indicates the process table grew in
+		// the meantime.
+		buf := make([]KinfoProc, n/SizeofKinfoProc)
+		if err := sysctl(mib, (*byte)(unsafe.Pointer(&buf[0])), &n, nil, 0); err != ENOMEM {
+			if 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)
+			}
 
-	// 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
+		}
+	}
 }
 
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