[PATCH v7 6/7] winedbg: Cache GDB qXfer command result for chunked fetching.

[Jinoh Kang]

GDB does not retrieve the result of a qXfer command at once; instead, it
issues a series of requests to obtain the result one "chunk" at a time,
and concatenates those chunks internally.  Each request contains offset
and length variables that specify which portion of the result shall be
retrieved.

Today, Winedbg handles this by generating the entire result data each
time a request is received and slicing out the requested range for the
response.  This is not only inefficient due to repeated computation,
but also prone to race condition since the result may change between
successive chunk requests due to the dynamic nature of some commands
such as "libraries" and "threads."

Fix this by cacheing the result into a buffer at the first request, and
use the buffer to serve successive chunk requests.  The cache is
invalidated when the remote requests a different object, or the debugger
reaches the end of the result cache buffer.

Signed-off-by: Jinoh Kang <jinoh.kang.kr at gmail.com>
---

Notes:
    v3 -> v4:
    - Refactor the code a bit
    - Revert to scanf() when parsing Xfer packet lines
    - Fix pointer variable declaration style
    
    v3 -> v4: Synchronize to prior modified patches.
    
    v4 -> v5:
    - Synchronize to prior modified patches.
    - Fix pointer variable declaration style.

 programs/winedbg/gdbproxy.c | 42 ++++++++++++++++++++++++++++---------
 1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/programs/winedbg/gdbproxy.c b/programs/winedbg/gdbproxy.c
index 0f5d9df35b8..c21fed43b48 100644
--- a/programs/winedbg/gdbproxy.c
+++ b/programs/winedbg/gdbproxy.c
@@ -931,15 +931,17 @@ static inline void packet_reply_register_hex_to(struct gdb_context* gdbctx, dbg_
     packet_reply_hex_to(gdbctx, cpu_register_ptr(gdbctx, ctx, idx), cpu_register_map[idx].length);
 }
 
-static void packet_reply_xfer(struct gdb_context* gdbctx, size_t off, size_t len)
+static void packet_reply_xfer(struct gdb_context* gdbctx, size_t off, size_t len, BOOL* more_p)
 {
+    BOOL more;
     size_t data_len, trunc_len;
 
     packet_reply_open(gdbctx);
     data_len = gdbctx->qxfer_buffer.len;
 
     /* check if off + len would overflow */
-    if (off < data_len && off + len < data_len)
+    more = off < data_len && off + len < data_len;
+    if (more)
         packet_reply_add(gdbctx, "m");
     else
         packet_reply_add(gdbctx, "l");
@@ -951,6 +953,8 @@ static void packet_reply_xfer(struct gdb_context* gdbctx, size_t off, size_t len
     }
 
     packet_reply_close(gdbctx);
+
+    *more_p = more;
 }
 
 /* =============================================== *
@@ -2071,6 +2075,7 @@ static enum packet_return packet_query(struct gdb_context* gdbctx)
         {
             enum packet_return result;
             int i;
+            BOOL more;
 
             for (i = 0; i < ARRAY_SIZE(qxfer_handlers); i++)
             {
@@ -2086,21 +2091,38 @@ static enum packet_return packet_query(struct gdb_context* gdbctx)
 
             TRACE("qXfer %s read %s %u,%u\n", debugstr_a(object_name), debugstr_a(annex), off, len);
 
-            gdbctx->qxfer_object_idx = i;
-            strcpy(gdbctx->qxfer_object_annex, annex);
+            if (off > 0 &&
+                gdbctx->qxfer_buffer.len > 0 &&
+                gdbctx->qxfer_object_idx == i &&
+                strcmp(gdbctx->qxfer_object_annex, annex) == 0)
+            {
+                result = packet_send_buffer;
+                TRACE("qXfer read result = %d (cached)\n", result);
+            }
+            else
+            {
+                reply_buffer_clear(&gdbctx->qxfer_buffer);
 
-            result = (*qxfer_handlers[i].handler)(gdbctx);
-            TRACE("qXfer read result = %d\n", result);
+                gdbctx->qxfer_object_idx = i;
+                strcpy(gdbctx->qxfer_object_annex, annex);
 
+                result = (*qxfer_handlers[i].handler)(gdbctx);
+                TRACE("qXfer read result = %d\n", result);
+            }
+
+            more = FALSE;
             if ((result & ~packet_last_f) == packet_send_buffer)
             {
-                packet_reply_xfer(gdbctx, off, len);
+                packet_reply_xfer(gdbctx, off, len, &more);
                 result = (result & packet_last_f) | packet_done;
             }
 
-            gdbctx->qxfer_object_idx = -1;
-            gdbctx->qxfer_object_annex[0] = '\0';
-            reply_buffer_clear(&gdbctx->qxfer_buffer);
+            if (!more)
+            {
+                gdbctx->qxfer_object_idx = -1;
+                gdbctx->qxfer_object_annex[0] = '\0';
+                reply_buffer_clear(&gdbctx->qxfer_buffer);
+            }
 
             return result;
         }
-- 
2.31.1