[PATCH v4 05/11] migration: Make qemu_get_buffer_at() thread-safe

[Aadeshveer Singh <aadeshveer07@gmail.com>]

Remove the internal QEMUFile error state modification from
qemu_get_buffer_at(). This function is called by two functions, both of
which already check for unexpected return values and handle their own
error reporting.

Removing this shared state modification makes qemu_get_buffer_at()
strictly thread-safe for concurrent disk reads, serving as a
preparatory change for the upcoming fast snapshot load feature.

Removed local error by passing errp to improve on error handling,
consequently also change caller to use error_prevent and not error_setg.

Set errp in case of f->last_error as function should set errp in case it
fails, and caller can simply use error_prepend without checking errp in
case of error.

Signed-off-by: Aadeshveer Singh <aadeshveer07@gmail.com>
---
 migration/qemu-file.c | 11 +++++------
 migration/qemu-file.h |  4 ++--
 migration/ram.c       |  6 +++---
 3 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index d5a48115bd..2feaeb8982 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -550,17 +550,16 @@ void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
 }
 
 
-size_t qemu_get_buffer_at(QEMUFile *f, uint8_t *buf, size_t buflen,
-                          off_t pos)
+size_t qemu_get_buffer_at(QEMUFile *f, uint8_t *buf, size_t buflen, off_t pos,
+                          Error **errp)
 {
-    Error *err = NULL;
-
     if (f->last_error) {
+        error_setg(errp, "Cannot read from file: stream is in error state %d",
+                   f->last_error);
         return 0;
     }
 
-    if (qio_channel_pread_all(f->ioc, buf, buflen, pos, &err) < 0) {
-        qemu_file_set_error_obj(f, -EIO, err);
+    if (qio_channel_pread_all(f->ioc, buf, buflen, pos, errp) < 0) {
         return 0;
     }
 
diff --git a/migration/qemu-file.h b/migration/qemu-file.h
index 8f824c124d..966766788d 100644
--- a/migration/qemu-file.h
+++ b/migration/qemu-file.h
@@ -76,8 +76,8 @@ void qemu_set_offset(QEMUFile *f, off_t off, int whence);
 off_t qemu_get_offset(QEMUFile *f);
 void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
                         off_t pos);
-size_t qemu_get_buffer_at(QEMUFile *f, uint8_t *buf, size_t buflen,
-                          off_t pos);
+size_t qemu_get_buffer_at(QEMUFile *f, uint8_t *buf, size_t buflen, off_t pos,
+                          Error **errp);
 
 QIOChannel *qemu_file_get_ioc(QEMUFile *file);
 int qemu_file_put_fd(QEMUFile *f, int fd);
diff --git a/migration/ram.c b/migration/ram.c
index 4728f14d73..967db7c0db 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -4127,7 +4127,7 @@ static bool read_ramblock_mapped_ram(QEMUFile *f, RAMBlock *block,
                                               block->pages_offset + offset);
             } else {
                 read = qemu_get_buffer_at(f, host, size,
-                                          block->pages_offset + offset);
+                                          block->pages_offset + offset, errp);
             }
 
             if (!read) {
@@ -4196,8 +4196,8 @@ static void parse_ramblock_mapped_ram(QEMUFile *f, RAMBlock *block,
     bitmap_size = BITS_TO_LONGS(num_pages) * sizeof(unsigned long);
 
     if (qemu_get_buffer_at(f, (uint8_t *)block->file_bmap, bitmap_size,
-                           header.bitmap_offset) != bitmap_size) {
-        error_setg(errp, "Error reading dirty bitmap");
+                           header.bitmap_offset, errp) != bitmap_size) {
+        error_prepend(errp, "Error reading dirty bitmap");
         return;
     }
 
-- 
2.55.0