[Qemu-devel] [PATCH v2 7/7] error: On abort, report where the error was created

[Markus Armbruster]

This is particularly useful when we abort in error_propagate(),
because there the stack backtrace doesn't lead to where the error was
created.  Looks like this:

    Unexpected error in parse_block_error_action() at 
/work/armbru/qemu/blockdev.c:322:
    qemu-system-x86_64: -drive if=none,werror=foo: 'foo' invalid write error 
action
    Aborted (core dumped)

Note: to get this example output, I monkey-patched drive_new() to pass
&error_abort to blockdev_init().

To keep the error handling boiler plate from growing even more, all
error_setFOO() become macros expanding into error_setFOO_internal()
with additional __FILE__, __LINE__, __func__ arguments.  Not exactly
pretty, but it works.

The macro trickery breaks down when you take the address of an
error_setFOO().  Fortunately, we do that in just one place: qemu-ga's
Windows VSS provider and requester DLL wants to call
error_setg_win32() through a function pointer "to avoid linking glib
to the DLL".  Use error_setg_win32_internal() there.  The use of the
function pointer is already wrapped in a macro, so the churn isn't
bad.

Code size increases by some 35KiB for me (0.7%).  Tolerable.  Could be
less if we passed relative rather than absolute source file names to
the compiler, or forwent reporting __func__.

Signed-off-by: Markus Armbruster <address@hidden>
---
 include/qapi/error.h        | 43 +++++++++++++++++++++++++++--------
 qga/vss-win32.c             |  2 +-
 qga/vss-win32/requester.cpp |  5 +++--
 qga/vss-win32/requester.h   |  6 +++--
 util/error.c                | 55 ++++++++++++++++++++++++++++++++-------------
 5 files changed, 81 insertions(+), 30 deletions(-)

diff --git a/include/qapi/error.h b/include/qapi/error.h
index 7d808e8..6285ade 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -104,16 +104,26 @@ ErrorClass error_get_class(const Error *err);
  * The new error's class is ERROR_CLASS_GENERIC_ERROR, and its
  * human-readable error message is made from printf-style @fmt, ...
  */
-void error_setg(Error **errp, const char *fmt, ...)
-    GCC_FMT_ATTR(2, 3);
+#define error_setg(errp, fmt, ...)                              \
+    error_setg_internal((errp), __FILE__, __LINE__, __func__,   \
+                        (fmt), ## __VA_ARGS__)
+void error_setg_internal(Error **errp,
+                         const char *src, int line, const char *func,
+                         const char *fmt, ...)
+    GCC_FMT_ATTR(5, 6);
 
 /*
  * Just like error_setg(), with @os_error info added to the message.
  * If @os_error is non-zero, ": " + strerror(os_error) is appended to
  * the human-readable error message.
  */
-void error_setg_errno(Error **errp, int os_error, const char *fmt, ...)
-    GCC_FMT_ATTR(3, 4);
+#define error_setg_errno(errp, os_error, fmt, ...)                      \
+    error_setg_errno_internal((errp), __FILE__, __LINE__, __func__,     \
+                              (os_error), (fmt), ## __VA_ARGS__)
+void error_setg_errno_internal(Error **errp,
+                               const char *fname, int line, const char *func,
+                               int os_error, const char *fmt, ...)
+    GCC_FMT_ATTR(6, 7);
 
 #ifdef _WIN32
 /*
@@ -121,8 +131,13 @@ void error_setg_errno(Error **errp, int os_error, const 
char *fmt, ...)
  * If @win32_error is non-zero, ": " + g_win32_error_message(win32_err)
  * is appended to the human-readable error message.
  */
-void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
-    GCC_FMT_ATTR(3, 4);
+#define error_setg_win32(errp, win32_err, fmt, ...)                     \
+    error_setg_win32_internal((errp), __FILE__, __LINE__, __func__,     \
+                              (win32_err), (fmt), ## __VA_ARGS__)
+void error_setg_win32_internal(Error **errp,
+                               const char *src, int line, const char *func,
+                               int win32_err, const char *fmt, ...)
+    GCC_FMT_ATTR(6, 7);
 #endif
 
 /*
@@ -143,7 +158,12 @@ void error_propagate(Error **dst_errp, Error *local_err);
 /*
  * Convenience function to report open() failure.
  */
-void error_setg_file_open(Error **errp, int os_errno, const char *filename);
+#define error_setg_file_open(errp, os_errno, filename)                  \
+    error_setg_file_open_internal((errp), __FILE__, __LINE__, __func__, \
+                                  (os_errno), (filename))
+void error_setg_file_open_internal(Error **errp,
+                                   const char *src, int line, const char *func,
+                                   int os_errno, const char *filename);
 
 /*
  * Return an exact copy of @err.
@@ -166,8 +186,13 @@ void error_report_err(Error *);
  * Note: use of error classes other than ERROR_CLASS_GENERIC_ERROR is
  * strongly discouraged.
  */
-void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
-    GCC_FMT_ATTR(3, 4);
+#define error_set(errp, err_class, fmt, ...)                    \
+    error_set_internal((errp), __FILE__, __LINE__, __func__,    \
+                       (err_class), (fmt), ## __VA_ARGS__)
+void error_set_internal(Error **errp,
+                        const char *src, int line, const char *func,
+                        ErrorClass err_class, const char *fmt, ...)
+    GCC_FMT_ATTR(6, 7);
 
 /*
  * Pass to error_setg() & friends to abort() on error.
diff --git a/qga/vss-win32.c b/qga/vss-win32.c
index d75d7bb..2142b49 100644
--- a/qga/vss-win32.c
+++ b/qga/vss-win32.c
@@ -150,7 +150,7 @@ void qga_vss_fsfreeze(int *nr_volume, Error **errp, bool 
freeze)
     const char *func_name = freeze ? "requester_freeze" : "requester_thaw";
     QGAVSSRequesterFunc func;
     ErrorSet errset = {
-        .error_setg_win32 = error_setg_win32,
+        .error_setg_win32 = error_setg_win32_internal,
         .errp = errp,
     };
 
diff --git a/qga/vss-win32/requester.cpp b/qga/vss-win32/requester.cpp
index aae0d5f..bb8cc2c 100644
--- a/qga/vss-win32/requester.cpp
+++ b/qga/vss-win32/requester.cpp
@@ -23,8 +23,9 @@
 /* Call QueryStatus every 10 ms while waiting for frozen event */
 #define VSS_TIMEOUT_EVENT_MSEC 10
 
-#define err_set(e, err, fmt, ...) \
-    ((e)->error_setg_win32((e)->errp, err, fmt, ## __VA_ARGS__))
+#define err_set(e, err, fmt, ...)                                       \
+    ((e)->error_setg_win32((e)->errp, __FILE__, __LINE__, __func__,     \
+                                    err, fmt, ## __VA_ARGS__))
 /* Bad idea, works only when (e)->errp != NULL: */
 #define err_is_set(e) ((e)->errp && *(e)->errp)
 /* To lift this restriction, error_propagate(), like we do in QEMU code */
diff --git a/qga/vss-win32/requester.h b/qga/vss-win32/requester.h
index 34be5c1..c3093cf 100644
--- a/qga/vss-win32/requester.h
+++ b/qga/vss-win32/requester.h
@@ -23,8 +23,10 @@ extern "C" {
 struct Error;
 
 /* Callback to set Error; used to avoid linking glib to the DLL */
-typedef void (*ErrorSetFunc)(struct Error **errp, int win32_err,
-                             const char *fmt, ...) GCC_FMT_ATTR(3, 4);
+typedef void (*ErrorSetFunc)(struct Error **errp,
+                             const char *src, int line, const char *func,
+                             int win32_err, const char *fmt, ...)
+    GCC_FMT_ATTR(6, 7);
 typedef struct ErrorSet {
     ErrorSetFunc error_setg_win32;
     struct Error **errp;        /* restriction: must not be null */
diff --git a/util/error.c b/util/error.c
index b842f20..1dcea2b 100644
--- a/util/error.c
+++ b/util/error.c
@@ -18,12 +18,23 @@ struct Error
 {
     char *msg;
     ErrorClass err_class;
+    const char *src, *func;
+    int line;
 };
 
 Error *error_abort;
 
-static void error_setv(Error **errp, ErrorClass err_class,
-                       const char *fmt, va_list ap)
+static void error_do_abort(Error *err)
+{
+    fprintf(stderr, "Unexpected error in %s() at %s:%d:\n",
+            err->func, err->src, err->line);
+    error_report_err(err);
+    abort();
+}
+
+static void error_setv(Error **errp,
+                       const char *src, int line, const char *func,
+                       ErrorClass err_class, const char *fmt, va_list ap)
 {
     Error *err;
     int saved_errno = errno;
@@ -36,10 +47,12 @@ static void error_setv(Error **errp, ErrorClass err_class,
     err = g_malloc0(sizeof(*err));
     err->msg = g_strdup_vprintf(fmt, ap);
     err->err_class = err_class;
+    err->src = src;
+    err->line = line;
+    err->func = func;
 
     if (errp == &error_abort) {
-        error_report_err(err);
-        abort();
+        error_do_abort(err);
     }
 
     *errp = err;
@@ -47,25 +60,31 @@ static void error_setv(Error **errp, ErrorClass err_class,
     errno = saved_errno;
 }
 
-void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
+void error_set_internal(Error **errp,
+                        const char *src, int line, const char *func,
+                        ErrorClass err_class, const char *fmt, ...)
 {
     va_list ap;
 
     va_start(ap, fmt);
-    error_setv(errp, err_class, fmt, ap);
+    error_setv(errp, src, line, func, err_class, fmt, ap);
     va_end(ap);
 }
 
-void error_setg(Error **errp, const char *fmt, ...)
+void error_setg_internal(Error **errp,
+                         const char *src, int line, const char *func,
+                         const char *fmt, ...)
 {
     va_list ap;
 
     va_start(ap, fmt);
-    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
+    error_setv(errp, src, line, func, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
     va_end(ap);
 }
 
-void error_setg_errno(Error **errp, int os_errno, const char *fmt, ...)
+void error_setg_errno_internal(Error **errp,
+                               const char *src, int line, const char *func,
+                              int os_errno, const char *fmt, ...)
 {
     va_list ap;
     char *msg;
@@ -76,7 +95,7 @@ void error_setg_errno(Error **errp, int os_errno, const char 
*fmt, ...)
     }
 
     va_start(ap, fmt);
-    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
+    error_setv(errp, src, line, func, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
     va_end(ap);
 
     if (os_errno != 0) {
@@ -88,14 +107,19 @@ void error_setg_errno(Error **errp, int os_errno, const 
char *fmt, ...)
     errno = saved_errno;
 }
 
-void error_setg_file_open(Error **errp, int os_errno, const char *filename)
+void error_setg_file_open_internal(Error **errp,
+                                   const char *src, int line, const char *func,
+                                   int os_errno, const char *filename)
 {
-    error_setg_errno(errp, os_errno, "Could not open '%s'", filename);
+    error_setg_errno_internal(errp, src, line, func, os_errno,
+                              "Could not open '%s'", filename);
 }
 
 #ifdef _WIN32
 
-void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
+void error_setg_win32_internal(Error **errp,
+                               const char *src, int line, const char *func,
+                               int win32_err, const char *fmt, ...)
 {
     va_list ap;
     char *msg1, *msg2;
@@ -105,7 +129,7 @@ void error_setg_win32(Error **errp, int win32_err, const 
char *fmt, ...)
     }
 
     va_start(ap, fmt);
-    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
+    error_setv(errp, src, line, func, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
     va_end(ap);
 
     if (win32_err != 0) {
@@ -158,8 +182,7 @@ void error_free(Error *err)
 void error_propagate(Error **dst_errp, Error *local_err)
 {
     if (local_err && dst_errp == &error_abort) {
-        error_report_err(local_err);
-        abort();
+        error_do_abort(local_err);
     } else if (dst_errp && !*dst_errp) {
         *dst_errp = local_err;
     } else if (local_err) {
-- 
1.9.3