improve daemon mode

[benoit-pierre]

Ensure fifo is created before forking: avoids possible race condition when accessing the fifo immediately after starting fbink.

---

This change is 

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[benoit-pierre]

Example, on kindle, with this patch to koreader.sh to test a workaround for fbink crashing (cf. koreader/koreader#15802):

--- a/platform/kindle/koreader.sh
+++ b/platform/kindle/koreader.sh
@@ -138,6 +138,11 @@ ko_update_check() {
         export FBINK_NAMED_PIPE="/tmp/koreader.fbink"
         rm -f "${FBINK_NAMED_PIPE}"
         FBINK_PID="$(/var/tmp/fbink --daemon 1 %KOREADER% -q -y -6 -P 0)"
+        # Open a handle to the fifo ourselves too: this prevent tar checkpointing
+        # calls from hanging when writing to the fifo if fbink crashed (even with
+        # no one reading from the fifo, its buffer should still be big enough to
+        # never be full either).
+        exec 3<>"${FBINK_NAMED_PIPE}"
         # NOTE: To avoid blowing up when an executable get truncated during use, we copy our binaries to the system's
         # tmpfs, and run them from there (c.f., #4602)...  This is most likely a side-effect of the weird fuse overlay
         # being used for /mnt/us (vs. the real vfat on /mnt/base-us), which we cannot use because it's been mounted
@@ -167,6 +172,9 @@ ko_update_check() {
             eips_print_bottom_centered "KOReader may fail to function properly" 1
         fi
         rm -f /tmp/package.index "${NEWUPDATE}" # always purge newupdate to prevent update loops
+        # Fifo cleanup: close our handle, and unlink the path too (in case fbink crashed).
+        exec 3>&-
+        rm -f "${FBINK_NAMED_PIPE}"
         unset FBINK_NAMED_PIPE FBINK_PID
         # Ensure everything is flushed to disk before we restart. This *will* stall for a while on slow storage!
         sync

Without the patch, or a sleep after starting, fbink fails to daemonize successfully.

[NiLuJe]

Huh. Okay!