[PATCH] util: open XML files before calling libxml2

[Daniel P. Berrangé <berrange@xxxxxxxxxx>]

Libxml2 has awful error reporting behaviour when reading files. When
we fail to load a file from the test driver we see:

  $ virsh -c test:///wibble.xml
  I/O warning : failed to load external entity "/wibble.xml"
  error: failed to connect to the hypervisor
  error: XML error: failed to parse xml document '/wibble.xml'

where the I/O warning line is something printed by libxml2 itself,
which also lacks any useful detail.

Switching to our own file reading code we can massively improve
things:

  $ ./build/tools/virsh -c test:///wibble.xml
  error: failed to connect to the hypervisor
  error: Failed to open file '/wibble.xml': No such file or directory

Using 10 MB as an upper limit on XML file size ought to be sufficient
for any XML files libvirt is reading.

Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
---
 src/util/virxml.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/util/virxml.c b/src/util/virxml.c
index a7b75fd7b3..f6b937b277 100644
--- a/src/util/virxml.c
+++ b/src/util/virxml.c
@@ -1133,6 +1133,7 @@ virXMLParseHelper(int domcode,
     xmlNodePtr rootnode;
     const char *docname;
     int parseFlags = XML_PARSE_NONET | XML_PARSE_NOWARNING;
+    g_autofree char *xmlStrPtr = NULL;
 
     if (filename)
         docname = filename;
@@ -1155,10 +1156,11 @@ virXMLParseHelper(int domcode,
     }
 
     if (filename) {
-        xml = xmlCtxtReadFile(pctxt, filename, NULL, parseFlags);
-    } else {
-        xml = xmlCtxtReadDoc(pctxt, BAD_CAST xmlStr, url, NULL, parseFlags);
+        if (virFileReadAll(filename, 1024*1024*10, &xmlStrPtr) < 0)
+            return NULL;
+        xmlStr = xmlStrPtr;
     }
+    xml = xmlCtxtReadDoc(pctxt, BAD_CAST xmlStr, url, NULL, parseFlags);
 
     if (!xml) {
         if (virGetLastErrorCode() == VIR_ERR_OK) {
-- 
2.45.2