To be able to add a common prefix or suffix to all trace output (e.g.
a timestamp or file:line of the caller), factor out common setup and
cleanup tasks of the trace* functions.

When adding a common prefix, it makes sense that the output of each trace
call starts on a new line. Add '\n' in case the caller forgot.

Note that this explicitly limits trace output to line-by-line, it is no
longer possible to trace-print just part of a line. Until now, this was
just an implicit assumption (trace-printing part of a line worked, but
messed up the trace file if multiple threads or processes were involved).

Thread-safety / inter-process-safety is also the reason why we need to do
the prefixing and suffixing in memory rather than issuing multiple write()
calls. Write_or_whine_pipe() / xwrite() is atomic unless the size exceeds
MAX_IO_SIZE (8MB, see wrapper.c). In case of trace_strbuf, this costs an
additional string copy (which should be irrelevant for performance in light
of actual file IO).

While we're at it, rename trace_strbuf's 'buf' argument, which suggests
that the function is modifying the buffer. Trace_strbuf() currently is the
only trace API that can print arbitrary binary data (without barfing on
'%' or stopping at '\0'), so 'data' seems more appropriate.

Signed-off-by: Karsten Blees <bl...@dcon.de>
---
 trace.c | 45 ++++++++++++++++++++++++++++++++++-----------
 trace.h |  2 +-
 2 files changed, 35 insertions(+), 12 deletions(-)

diff --git a/trace.c b/trace.c
index b7ca51b..c920429 100644
--- a/trace.c
+++ b/trace.c
@@ -77,17 +77,37 @@ static void do_trace_print(const char *key, const struct 
strbuf *buf)
                close(fd);
 }
 
+static int prepare_trace_line(const char *key, struct strbuf *buf)
+{
+       if (!trace_want(key))
+               return 0;
+
+       set_try_to_free_routine(NULL);  /* is never reset */
+
+       /* add line prefix here */
+
+       return 1;
+}
+
+static void print_trace_line(const char *key, struct strbuf *buf)
+{
+       /* append newline if missing */
+       if (buf->len && buf->buf[buf->len - 1] != '\n')
+               strbuf_addch(buf, '\n');
+
+       do_trace_print(key, buf);
+       strbuf_release(buf);
+}
+
 static void trace_vprintf(const char *key, const char *format, va_list ap)
 {
        struct strbuf buf = STRBUF_INIT;
 
-       if (!trace_want(key))
+       if (!prepare_trace_line(key, &buf))
                return;
 
-       set_try_to_free_routine(NULL);  /* is never reset */
        strbuf_vaddf(&buf, format, ap);
-       do_trace_print(key, &buf);
-       strbuf_release(&buf);
+       print_trace_line(key, &buf);
 }
 
 void trace_printf_key(const char *key, const char *format, ...)
@@ -106,9 +126,15 @@ void trace_printf(const char *format, ...)
        va_end(ap);
 }
 
-void trace_strbuf(const char *key, const struct strbuf *buf)
+void trace_strbuf(const char *key, const struct strbuf *data)
 {
-       do_trace_print(key, buf);
+       struct strbuf buf = STRBUF_INIT;
+
+       if (!prepare_trace_line(key, &buf))
+               return;
+
+       strbuf_addbuf(&buf, data);
+       print_trace_line(key, &buf);
 }
 
 void trace_argv_printf(const char **argv, const char *format, ...)
@@ -116,18 +142,15 @@ void trace_argv_printf(const char **argv, const char 
*format, ...)
        struct strbuf buf = STRBUF_INIT;
        va_list ap;
 
-       if (!trace_want("GIT_TRACE"))
+       if (!prepare_trace_line("GIT_TRACE", &buf))
                return;
 
-       set_try_to_free_routine(NULL);  /* is never reset */
        va_start(ap, format);
        strbuf_vaddf(&buf, format, ap);
        va_end(ap);
 
        sq_quote_argv(&buf, argv, 0);
-       strbuf_addch(&buf, '\n');
-       do_trace_print("GIT_TRACE", &buf);
-       strbuf_release(&buf);
+       print_trace_line("GIT_TRACE", &buf);
 }
 
 static const char *quote_crnl(const char *path)
diff --git a/trace.h b/trace.h
index 8fea50b..e03db2f 100644
--- a/trace.h
+++ b/trace.h
@@ -12,6 +12,6 @@ extern void trace_repo_setup(const char *prefix);
 extern int trace_want(const char *key);
 __attribute__((format (printf, 2, 3)))
 extern void trace_printf_key(const char *key, const char *format, ...);
-extern void trace_strbuf(const char *key, const struct strbuf *buf);
+extern void trace_strbuf(const char *key, const struct strbuf *data);
 
 #endif /* TRACE_H */
-- 
1.9.2.msysgit.0.501.gaeecf09

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