This is version 2 of [0].  It adds some comments explaining handling
of NUL bytes in strings and moves a few variable assignments closer to
their uses.

[0] id:1343183693-17134-1-git-send-email-amdragon@mit.edu

The diff from version 1 follows.  (Note that date was already assigned
where it was used in version 1, in addition to being assigned at the
beginning of format_message_json; hence the diff diff only shows it
being removed).

diff --git a/notmuch-show.c b/notmuch-show.c
index f5bc098..3556293 100644
--- a/notmuch-show.c
+++ b/notmuch-show.c
@@ -105,6 +105,8 @@ _get_one_line_summary (const void *ctx, notmuch_message_t *message)
 			    from, relative_date, tags);
 }
 
+/* Emit a sequence of key/value pairs for the metadata of message.
+ * The caller should begin a map before calling this. */
 static void
 format_message_json (sprinter_t *sp, notmuch_message_t *message)
 {
@@ -113,9 +115,6 @@ format_message_json (sprinter_t *sp, notmuch_message_t *message)
     time_t date;
     const char *relative_date;
 
-    date = notmuch_message_get_date (message);
-    relative_date = notmuch_time_relative_date (local, date);
-
     sp->map_key (sp, "id");
     sp->string (sp, notmuch_message_get_message_id (message));
 
@@ -133,6 +132,7 @@ format_message_json (sprinter_t *sp, notmuch_message_t *message)
     sp->integer (sp, date);
 
     sp->map_key (sp, "date_relative");
+    relative_date = notmuch_time_relative_date (local, date);
     sp->string (sp, relative_date);
 
     sp->map_key (sp, "tags");
diff --git a/sprinter-json.c b/sprinter-json.c
index 2587ca6..c9b6835 100644
--- a/sprinter-json.c
+++ b/sprinter-json.c
@@ -88,6 +88,11 @@ json_end (struct sprinter *sp)
 	fputc ('\n', spj->stream);
 }
 
+/* This implementation supports embedded NULs as allowed by the JSON
+ * specification and Unicode.  Support for *parsing* embedded NULs
+ * varies, but is generally not a problem outside of C-based parsers
+ * (Python's json module and Emacs' json.el take embedded NULs in
+ * stride). */
 static void
 json_string_len (struct sprinter *sp, const char *val, size_t len)
 {
diff --git a/sprinter.h b/sprinter.h
index 826a852..5f43175 100644
--- a/sprinter.h
+++ b/sprinter.h
@@ -23,9 +23,11 @@ typedef struct sprinter {
      */
     void (*end) (struct sprinter *);
 
-    /* Print one string/integer/boolean/null element (possibly inside a
-     * list or map, followed or preceded by separators).
-     * For string, the char * must be UTF-8 encoded.
+    /* Print one string/integer/boolean/null element (possibly inside
+     * a list or map, followed or preceded by separators).  For string
+     * and string_len, the char * must be UTF-8 encoded.  string_len
+     * allows non-terminated strings and strings with embedded NULs
+     * (though the handling of the latter is format-dependent).
      */
     void (*string) (struct sprinter *, const char *);
     void (*string_len) (struct sprinter *, const char *, size_t);