Fix deadlock in case test performs lots of checks without yielding

Before commit 6c5c8b7 ("Print luatest logs to stdout when unified
log is disabled"), luatest implicitly configured the logger to operate
in the non-blocking mode by using a pipe for the logs destination.
Now, it uses a file albeit the file actually points to a pipe file
descriptor. Tarantool thinks it's a regular file and writes all logs in
the blocking mode. This leads to a deadlock if a test performs a lot
of checks (which are logged) without yielding execution to the fiber
processing logs because the size of a pipe is quite limited. Let's fix
this issue by writing all logs through a pipe, as we did before. Let's
also explicitly enable non-blocking logging to avoid breakages like this
in future.

The fix isn't ideal because it may lead to dropping logs. We should
probably handle logs emitted by the luatest runner in a separate
process, like we do for processes spawned in tests.

Closes #416

Author: locker

luatest/log.lua

@@ -42,7 +42,12 @@ function log.initialize(options)
     output_beautifier:enable()
 
     -- Redirect all logs to the pipe created by OutputBeautifier.
-    local log_cfg = string.format('/dev/fd/%d', output_beautifier.pipes.stdout[1])
+    --
+    -- Write through a pipe to enable non-blocking mode. Required to prevent
+    -- a deadlock in case a test performs a lot of checks without yielding
+    -- execution to the fiber processing logs (gh-416).
+    local log_cfg = string.format('| cat > /dev/fd/%d',
+                                  output_beautifier.pipes.stdout[1])
 
     -- Logging cannot be initialized without configuring the `box` engine
     -- on a version less than 2.5.1 (see more details at [1]). Otherwise,
@@ -59,7 +64,10 @@ function log.initialize(options)
         -- Initialize logging for luatest runner.
         -- The log format will be as follows:
         --     YYYY-MM-DD HH:MM:SS.ZZZ [ID] main/.../luatest I> ...
-        require('log').cfg{log = log_cfg}
+        require('log').cfg{
+            log = log_cfg,
+            nonblock = true,
+        }
     end
 
     is_initialized = true