[Libguestfs] [nbdkit PATCH] log: Allow user option of appending to log

[Eric Blake]

Always truncating a log can wipe out useful history. Add an
option to change this.

Signed-off-by: Eric Blake <eblake at redhat.com>
---

Hmm - logappend=false enables appending. Maybe we need a common
utility function for parsing bool values (parse 0 and case-insensitive
"off" or "false" as off, all others as true, or even report values
that can't be parsed reasonably)

 filters/log/nbdkit-log-filter.pod | 5 +++--
 filters/log/log.c                 | 8 +++++++-
 tests/test-log.sh                 | 6 +++++-
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/filters/log/nbdkit-log-filter.pod b/filters/log/nbdkit-log-filter.pod
index 0903329..88b9f9b 100644
--- a/filters/log/nbdkit-log-filter.pod
+++ b/filters/log/nbdkit-log-filter.pod
@@ -4,7 +4,7 @@ nbdkit-log-filter - nbdkit log filter

 =head1 SYNOPSIS

- nbdkit --filter=log plugin logfile=FILE [plugin-args...]
+ nbdkit --filter=log plugin logfile=FILE [logappend=VAL] [plugin-args...]

 =head1 DESCRIPTION

@@ -22,7 +22,8 @@ work even when nbdkit is run as a daemon.

 The nbdkit-log-filter requires a single parameter C<logfile> which
 specifies the path of the file to use for logging.  If the file
-already exists, it will be truncated.
+already exists, it will be truncated unless the C<logappend> parameter
+was specified with any non-empty value other than "0".

 =head1 EXAMPLES

diff --git a/filters/log/log.c b/filters/log/log.c
index 2079084..e6ed3b4 100644
--- a/filters/log/log.c
+++ b/filters/log/log.c
@@ -43,6 +43,7 @@
 #include <pthread.h>
 #include <sys/time.h>
 #include <assert.h>
+#include <stdbool.h>

 #include <nbdkit-filter.h>

@@ -51,6 +52,7 @@
 static uint64_t connections;
 static char *logfilename;
 static FILE *logfile;
+static bool append;
 static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

 static void
@@ -75,6 +77,10 @@ log_config (nbdkit_next_config *next, void *nxdata,
     }
     return 0;
   }
+  if (strcmp (key, "logappend") == 0) {
+    append = value[0] && (value[0] != '0' || value[1]);
+    return 0;
+  }
   return next (nxdata, key, value);
 }

@@ -86,7 +92,7 @@ log_config_complete (nbdkit_next_config_complete *next, void *nxdata)
     nbdkit_error ("missing logfile= parameter for the log filter");
     return -1;
   }
-  logfile = fopen (logfilename, "w");
+  logfile = fopen (logfilename, append ? "a" : "w");
   if (!logfile) {
     nbdkit_error ("fopen: %m");
     return -1;
diff --git a/tests/test-log.sh b/tests/test-log.sh
index f0eacb7..b30d20e 100755
--- a/tests/test-log.sh
+++ b/tests/test-log.sh
@@ -45,7 +45,9 @@ if ! qemu-io -f raw -c 'w 1M 2M' log.img; then
 fi

 # Run nbdkit with logging enabled to file.
-start_nbdkit -P log.pid -U log.sock --filter=log file log.img logfile=log.log
+echo '# My log' > log.log
+start_nbdkit -P log.pid -U log.sock --filter=log file log.img \
+	     logfile=log.log logappend=1

 # For easier debugging, dump the final log files before removing them
 # on exit.
@@ -61,6 +63,8 @@ cleanup_fn cleanup
 qemu-io -f raw -c 'w -P 11 1M 2M' 'nbd+unix://?socket=log.sock'
 qemu-io -r -f raw -c 'r -P 11 2M 1M' 'nbd+unix://?socket=log.sock'

+# The log should have been appended, preserving our marker.
+grep '# My log' log.log
 # The log should show a write on connection 1, and read on connection 2.
 grep 'connection=1 Write id=1 offset=0x100000 count=0x200000 ' log.log
 grep 'connection=2 Read id=1 offset=0x200000 count=0x100000 ' log.log
-- 
2.17.2