fix(export): hash stanza_id with content instead of trusting it as sole dedup key

src/database_export.c

@@ -90,18 +90,21 @@ _close_temp_sqlite(int opened)
 // =========================================================================
 
 // Returns a g_strdup'd key — callers free with g_free (gchar* convention).
+//
+// stanza_id is mixed into the hash but is NOT used as the sole key, since
+// some clients (older Pidgin, Adium) reuse incremental ids per session and
+// servers echo them back unchanged. Trusting stanza_id alone silently drops
+// legitimate distinct messages on collision. By hashing id + timestamp +
+// from_jid + body together we preserve id as a disambiguator without making
+// it a single point of failure: true exact duplicates still dedup, but two
+// messages that share an id with different content (or different timestamps)
+// stay distinct.
 static gchar*
 _make_dedup_key(const char* stanza_id, const char* timestamp, const char* from_jid, const char* body)
 {
-    if (stanza_id && stanza_id[0] != '\0')
-        return g_strdup(stanza_id);
-
-    // Fallback for messages without stanza-id: SHA-256 over the FULL body,
-    // not the first 256 bytes. The previous prefix-only hash collided on
-    // legitimate near-identical templates (signatures, code blocks, paste-
-    // bombs) — second occurrence was silently dropped on import. Hashing
-    // whole body is microseconds of extra work per row.
     GChecksum* sum = g_checksum_new(G_CHECKSUM_SHA256);
+    g_checksum_update(sum, (const guchar*)(stanza_id ? stanza_id : ""), -1);
+    g_checksum_update(sum, (const guchar*)"|", 1);
     g_checksum_update(sum, (const guchar*)(timestamp ? timestamp : ""), -1);
     g_checksum_update(sum, (const guchar*)"|", 1);
     g_checksum_update(sum, (const guchar*)(from_jid ? from_jid : ""), -1);