Commit 567bcbc2 introduced support for storing various headers in
document values.  However, doing so in a backwards-compatible way
meant that genuinely empty header values could not be distinguished
from the old behavior of not storing the headers at all, so these
required parsing the original message.

Now that we have database features, new databases can declare that all
messages have header values, so if we have this feature flag, we can
use the stored header value even if it's the empty string.

This requires slight cleanup to notmuch_message_get_header, since the
code previously couldn't distinguish between empty headers and headers
that are never stored in the database (previously this distinction
didn't matter).
---
 lib/message.cc | 45 +++++++++++++++++++++++++++------------------
 1 file changed, 27 insertions(+), 18 deletions(-)

diff --git a/lib/message.cc b/lib/message.cc
index e6a5a5a..4fc427f 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -412,26 +412,35 @@ _notmuch_message_ensure_message_file (notmuch_message_t 
*message)
 const char *
 notmuch_message_get_header (notmuch_message_t *message, const char *header)
 {
-    try {
-           std::string value;
-
-           /* Fetch header from the appropriate xapian value field if
-            * available */
-           if (strcasecmp (header, "from") == 0)
-               value = message->doc.get_value (NOTMUCH_VALUE_FROM);
-           else if (strcasecmp (header, "subject") == 0)
-               value = message->doc.get_value (NOTMUCH_VALUE_SUBJECT);
-           else if (strcasecmp (header, "message-id") == 0)
-               value = message->doc.get_value (NOTMUCH_VALUE_MESSAGE_ID);
-
-           if (!value.empty())
+    Xapian::valueno slot = Xapian::BAD_VALUENO;
+
+    /* Fetch header from the appropriate xapian value field if
+     * available */
+    if (strcasecmp (header, "from") == 0)
+       slot = NOTMUCH_VALUE_FROM;
+    else if (strcasecmp (header, "subject") == 0)
+       slot = NOTMUCH_VALUE_SUBJECT;
+    else if (strcasecmp (header, "message-id") == 0)
+       slot = NOTMUCH_VALUE_MESSAGE_ID;
+
+    if (slot != Xapian::BAD_VALUENO) {
+       try {
+           std::string value = message->doc.get_value (slot);
+
+           /* If we have NOTMUCH_FEATURE_FROM_SUBJECT_ID_VALUES, then
+            * empty values indicate empty headers.  If we don't, then
+            * it could just mean we didn't record the header. */
+           if ((message->notmuch->features &
+                NOTMUCH_FEATURE_FROM_SUBJECT_ID_VALUES) ||
+               ! value.empty())
                return talloc_strdup (message, value.c_str ());

-    } catch (Xapian::Error &error) {
-       fprintf (stderr, "A Xapian exception occurred when reading header: 
%s\n",
-                error.get_msg().c_str());
-       message->notmuch->exception_reported = TRUE;
-       return NULL;
+       } catch (Xapian::Error &error) {
+           fprintf (stderr, "A Xapian exception occurred when reading header: 
%s\n",
+                    error.get_msg().c_str());
+           message->notmuch->exception_reported = TRUE;
+           return NULL;
+       }
     }

     /* Otherwise fall back to parsing the file */
-- 
2.0.0

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