lib/repo: Minor fixes around min-free-space

Summary:
* Remove a useless if condition in prepare_transaction()
* Fix glnx_throw error propagation
* Integer overflow check while parsing min-free-space-size config
* Documentation fixes

Closes: #1663
Approved by: jlebon

src/libostree/ostree-repo.c

@@ -2670,7 +2670,7 @@ min_free_space_size_validate_and_convert (OstreeRepo    *self,
 
   g_autoptr(GMatchInfo) match = NULL;
   if (!g_regex_match (regex, min_free_space_size_str, 0, &match))
-    return glnx_prefix_error (error, "Failed to parse min-free-space-size parameter: '%s'", min_free_space_size_str);
+    return glnx_throw (error, "Failed to match '^[0-9]+[GMT]B$'");
 
   g_autofree char *size_str = g_match_info_fetch (match, 1);
   g_autofree char *unit = g_match_info_fetch (match, 2);
@@ -2691,7 +2691,11 @@ min_free_space_size_validate_and_convert (OstreeRepo    *self,
         g_assert_not_reached ();
     }
 
-  self->min_free_space_mb = g_ascii_strtoull (size_str, NULL, 10) << shifts;
+  guint64 min_free_space = g_ascii_strtoull (size_str, NULL, 10);
+  if (shifts > 0 && g_bit_nth_lsf (min_free_space, 63 - shifts) != -1)
+    return glnx_throw (error, "Integer overflow detected");
+
+  self->min_free_space_mb = min_free_space << shifts;
 
   return TRUE;
 }
@@ -2829,7 +2833,7 @@ reload_core_config (OstreeRepo          *self,
 
         /* Validate the string and convert the size to MBs */
         if (!min_free_space_size_validate_and_convert (self, min_free_space_size_str, error))
-          return glnx_throw (error, "Invalid min-free-space-size '%s'", min_free_space_size_str);
+          return glnx_prefix_error (error, "Invalid min-free-space-size '%s'", min_free_space_size_str);
       }
     else
       {