libxl: Fix leak of ctx->lock

A mutex created with pthread_mutex_init, like ctx->lock, may need to
be destroyed with pthread_mutex_destroy.

Also, previously, if libxl__init_recursive_mutex failed, the nascent
ctx would be leaked.  Add some comments which will hopefully make
these kind of mistakes less likely in future.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

tools/libxl/libxl.c

@@ -39,10 +39,7 @@ int libxl_ctx_alloc(libxl_ctx **pctx, int version,
     memset(ctx, 0, sizeof(libxl_ctx));
     ctx->lg = lg;
 
-    if (libxl__init_recursive_mutex(ctx, &ctx->lock) < 0) {
-        LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "Failed to initialize mutex");
-        return ERROR_FAIL;
-    }
+    /* First initialise pointers (cannot fail) */
 
     LIBXL_TAILQ_INIT(&ctx->occurred);
 
@@ -61,6 +58,16 @@ int libxl_ctx_alloc(libxl_ctx **pctx, int version,
     LIBXL_TAILQ_INIT(&ctx->death_list);
     libxl__ev_xswatch_init(&ctx->death_watch);
 
+    /* The mutex is special because we can't idempotently destroy it */
+
+    if (libxl__init_recursive_mutex(ctx, &ctx->lock) < 0) {
+        LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "Failed to initialize mutex");
+        free(ctx);
+        ctx = 0;
+    }
+
+    /* Now ctx is safe for ctx_free; failures simply set rc and "goto out" */
+
     rc = libxl__poller_init(ctx, &ctx->poller_app);
     if (rc) goto out;
 
@@ -150,6 +157,8 @@ int libxl_ctx_free(libxl_ctx *ctx)
 
     discard_events(&ctx->occurred);
 
+    pthread_mutex_destroy(&ctx->lock);
+
     GC_FREE;
     free(ctx);
     return 0;