Report original error when QMP probing fails with new QEMU

If probing capabilities via QMP fails, we now have a check
that prevents us falling back to -help parsing. Unfortunately
the error message

  "Failed to probe capabilities for /usr/bin/qemu-kvm:
   unsupported configuration: QEMU 2.1.2 is too new for help parsing"

is proving rather unhelpful to the user. We need to be telling
them why QMP failed (the root cause), rather than they can't
use -help (the side effect).

To do this we should capture stderr during QMP probing, and
if -help parsing then sees a new QEMU version, we know that
QMP should have worked, and so we can show the messages from
stderr. The message thus becomes

  "Failed to probe capabilities for /usr/bin/qemu-kvm:
   internal error: QEMU / QMP failed: Could not access
   KVM kernel module: No such file or directory
   failed to initialize KVM: No such file or directory"

src/qemu/qemu_capabilities.c

@@ -1331,7 +1331,8 @@ int virQEMUCapsParseHelpStr(const char *qemu,
                             unsigned int *version,
                             bool *is_kvm,
                             unsigned int *kvm_version,
-                            bool check_yajl)
+                            bool check_yajl,
+                            const char *qmperr)
 {
     unsigned major, minor, micro;
     const char *p = help;
@@ -1392,9 +1393,15 @@ int virQEMUCapsParseHelpStr(const char *qemu,
      * using QMP probing.
      */
     if (*version >= 1002000) {
+        if (qmperr && *qmperr) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("QEMU / QMP failed: %s"),
+                           qmperr);
+        } else {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                            _("QEMU %u.%u.%u is too new for help parsing"),
                            major, minor, micro);
+        }
         goto cleanup;
     }
 
@@ -2967,7 +2974,7 @@ virQEMUCapsInitCached(virQEMUCapsPtr qemuCaps, const char *cacheDir)
 #define QEMU_SYSTEM_PREFIX "qemu-system-"
 
 static int
-virQEMUCapsInitHelp(virQEMUCapsPtr qemuCaps, uid_t runUid, gid_t runGid)
+virQEMUCapsInitHelp(virQEMUCapsPtr qemuCaps, uid_t runUid, gid_t runGid, const char *qmperr)
 {
     virCommandPtr cmd = NULL;
     bool is_kvm;
@@ -2998,7 +3005,8 @@ virQEMUCapsInitHelp(virQEMUCapsPtr qemuCaps, uid_t runUid, gid_t runGid)
                                 &qemuCaps->version,
                                 &is_kvm,
                                 &qemuCaps->kvmVersion,
-                                false) < 0)
+                                false,
+                                qmperr) < 0)
         goto cleanup;
 
     /* x86_64 and i686 support PCI-multibus on all machine types
@@ -3249,7 +3257,8 @@ static int
 virQEMUCapsInitQMP(virQEMUCapsPtr qemuCaps,
                    const char *libDir,
                    uid_t runUid,
-                   gid_t runGid)
+                   gid_t runGid,
+                   char **qmperr)
 {
     int ret = -1;
     virCommandPtr cmd = NULL;
@@ -3311,13 +3320,16 @@ virQEMUCapsInitQMP(virQEMUCapsPtr qemuCaps,
     virCommandSetGID(cmd, runGid);
     virCommandSetUID(cmd, runUid);
 
+    virCommandSetErrorBuffer(cmd, qmperr);
+
     /* Log, but otherwise ignore, non-zero status.  */
     if (virCommandRun(cmd, &status) < 0)
         goto cleanup;
 
     if (status != 0) {
         ret = 0;
-        VIR_DEBUG("QEMU %s exited with status %d", qemuCaps->binary, status);
+        VIR_DEBUG("QEMU %s exited with status %d: %s",
+                  qemuCaps->binary, status, *qmperr);
         goto cleanup;
     }
 
@@ -3366,6 +3378,8 @@ virQEMUCapsInitQMP(virQEMUCapsPtr qemuCaps,
             VIR_ERROR(_("Failed to kill process %lld: %s"),
                       (long long) pid,
                       virStrerror(errno, ebuf, sizeof(ebuf)));
+
+        VIR_FREE(*qmperr);
     }
     if (pidfile) {
         unlink(pidfile);
@@ -3406,6 +3420,7 @@ virQEMUCapsPtr virQEMUCapsNewForBinary(const char *binary,
     virQEMUCapsPtr qemuCaps;
     struct stat sb;
     int rv;
+    char *qmperr = NULL;
 
     if (!(qemuCaps = virQEMUCapsNew()))
         goto error;
@@ -3436,13 +3451,13 @@ virQEMUCapsPtr virQEMUCapsNewForBinary(const char *binary,
         goto error;
 
     if (rv == 0) {
-        if (virQEMUCapsInitQMP(qemuCaps, libDir, runUid, runGid) < 0) {
+        if (virQEMUCapsInitQMP(qemuCaps, libDir, runUid, runGid, &qmperr) < 0) {
             virQEMUCapsLogProbeFailure(binary);
             goto error;
         }
 
         if (!qemuCaps->usedQMP &&
-            virQEMUCapsInitHelp(qemuCaps, runUid, runGid) < 0) {
+            virQEMUCapsInitHelp(qemuCaps, runUid, runGid, qmperr) < 0) {
             virQEMUCapsLogProbeFailure(binary);
             goto error;
         }
@@ -3451,9 +3466,11 @@ virQEMUCapsPtr virQEMUCapsNewForBinary(const char *binary,
             goto error;
     }
 
+    VIR_FREE(qmperr);
     return qemuCaps;
 
  error:
+    VIR_FREE(qmperr);
     virObjectUnref(qemuCaps);
     qemuCaps = NULL;
     return NULL;

src/qemu/qemu_capabilities.h

@@ -308,7 +308,8 @@ int virQEMUCapsParseHelpStr(const char *qemu,
                             unsigned int *version,
                             bool *is_kvm,
                             unsigned int *kvm_version,
-                            bool check_yajl);
+                            bool check_yajl,
+                            const char *qmperr);
 /* Only for use by test suite */
 int virQEMUCapsParseDeviceStr(virQEMUCapsPtr qemuCaps, const char *str);
 

tests/qemuhelptest.c

@@ -59,7 +59,7 @@ static int testHelpStrParsing(const void *data)
         goto cleanup;
 
     if (virQEMUCapsParseHelpStr("QEMU", help, flags,
-                                &version, &is_kvm, &kvm_version, false) == -1) {
+                                &version, &is_kvm, &kvm_version, false, NULL) == -1) {
         if (info->error && virGetLastError()->code == info->error)
             ret = 0;
         goto cleanup;