[Qemu-devel] [PATCH v2 1/2] qga: flush explicitly when needed

[marcandre.lureau@redhat.com]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

According to the specification:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html

"the application shall ensure that output is not directly followed by
input without an intervening call to fflush() or to a file positioning
function (fseek(), fsetpos(), or rewind()), and input is not directly
followed by output without an intervening call to a file positioning
function, unless the input operation encounters end-of-file."

Without this change, a write() followed by a read() may lose the
previously written content, as shown in the following test.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1210246

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 qga/commands-posix.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 0ebd473..d0228ce 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -219,6 +219,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 typedef struct GuestFileHandle {
     uint64_t id;
     FILE *fh;
+    bool writing;
     QTAILQ_ENTRY(GuestFileHandle) next;
 } GuestFileHandle;
 
@@ -460,6 +461,17 @@ struct GuestFileRead *qmp_guest_file_read(int64_t handle, bool has_count,
     }
 
     fh = gfh->fh;
+
+    /* explicitly flush when switching from writing to reading */
+    if (gfh->writing) {
+        int ret = fflush(fh);
+        if (ret == EOF) {
+            error_setg_errno(errp, errno, "failed to flush file");
+            return NULL;
+        }
+        gfh->writing = false;
+    }
+
     buf = g_malloc0(count+1);
     read_count = fread(buf, 1, count, fh);
     if (ferror(fh)) {
@@ -496,6 +508,16 @@ GuestFileWrite *qmp_guest_file_write(int64_t handle, const char *buf_b64,
     }
 
     fh = gfh->fh;
+
+    if (!gfh->writing) {
+        int ret = fseek(fh, 0, SEEK_CUR);
+        if (ret == -1) {
+            error_setg_errno(errp, errno, "failed to seek file");
+            return NULL;
+        }
+        gfh->writing = true;
+    }
+
     buf = g_base64_decode(buf_b64, &buf_len);
 
     if (!has_count) {
-- 
2.5.0