[PATCH] Add check for binary existing in machine type probe

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

When probing machine types if the QEMU binary does not exist
we get a hard to diagnose error, due to the execve() in the
child failing

error: internal error Child process exited with status 1.

Add an explicit check so that we get

error: Cannot find QEMU binary /usr/libexec/qem3u-kvm: No such file or directory

* src/qemu/qemu_capabilities.c: Check for QEMU binary
---
 src/qemu/qemu_capabilities.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index 37a97aa..08c46be 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -172,6 +172,15 @@ qemuCapsProbeMachineTypes(const char *binary,
     int ret = -1;
     virCommandPtr cmd;
 
+    /* Make sure the binary we are about to try exec'ing exists.
+     * Technically we could catch the exec() failure, but that's
+     * in a sub-process so it's hard to feed back a useful error.
+     */
+    if (access(binary, X_OK) < 0) {
+        virReportSystemError(errno, _("Cannot find QEMU binary %s"), binary);
+        return -1;
+    }
+
     cmd = virCommandNewArgList(binary, "-M", "?", NULL);
     virCommandAddEnvPassCommon(cmd);
     virCommandSetOutputBuffer(cmd, &output);
-- 
1.7.3.5

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