set/show python dont-write-bytecode fixes

GDB uses the environment variable PYTHONDONTWRITEBYTECODE to
determine whether or not to write the result of byte-compiling
python modules when the "python dont-write-bytecode" setting
is "auto".  Simon noticed that GDB's implementation doesn't
follow the Python documentation.

At present, GDB only checks for the existence of this environment
variable.  That is not sufficient though.  Regarding
PYTHONDONTWRITEBYTECODE, this document...

    https://docs.python.org/3/using/cmdline.html

...says:

    If this is set to a non-empty string, Python won't try to write
    .pyc files on the import of source modules.

This commit fixes GDB's handling of PYTHONDONTWRITEBYTECODE by adding
an empty string check.

This commit also corrects the set/show command documentation for
"python dont-write-bytecode".  The current doc was just a copy
of that for set/show python ignore-environment.

During his review of an earlier version of this patch, Eli Zaretskii
asked that the help text that I proposed for "set/show python
dont-write-bytecode" be expanded.  I've done that in addition to
clarifying the documentation of this option in the GDB manual.

2 files changed, 32 insertions, 9 deletions

diff --git a/gdb/doc/python.texi b/gdb/doc/python.texi
index eeb847aeaa..cdf7db9160 100644
--- a/gdb/doc/python.texi
+++ b/gdb/doc/python.texi
@@ -134,9 +134,14 @@ initialized early during @value{GDBN}'s startup process, then this
 option must be placed into the early initialization file
 (@pxref{Initialization Files}) to have the desired effect.
 
-By default this option is set to @samp{auto}, in this mode Python will
-check the environment variable @env{PYTHONDONTWRITEBYTECODE} to see
+By default this option is set to @samp{auto}.  In this mode, provided
+the @code{python ignore-environment} setting is @samp{off}, the
+environment variable @env{PYTHONDONTWRITEBYTECODE} is examined to see
 if it should write out byte-code or not.
+@env{PYTHONDONTWRITEBYTECODE} is considered to be off/disabled either
+when set to the empty string or when the environment variable doesn't
+exist.  All other settings, including those which don't seem to make
+sense, indicate that it's on/enabled.
 
 This option is equivalent to passing @option{-B} to the real
 @command{python} executable.
diff --git a/gdb/python/python.c b/gdb/python/python.c
index c7d4157b70..7997bee2c6 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -1866,8 +1866,15 @@ python_write_bytecode ()
   int wbc = 0;
 
   if (python_dont_write_bytecode == AUTO_BOOLEAN_AUTO)
-    wbc = (!python_ignore_environment
-	    && getenv ("PYTHONDONTWRITEBYTECODE") != nullptr) ? 0 : 1;
+    {
+      if (python_ignore_environment)
+	wbc = 1;
+      else
+	{
+	  const char *pdwbc = getenv ("PYTHONDONTWRITEBYTECODE");
+	  wbc = (pdwbc == nullptr || pdwbc[0] == '\0') ? 1 : 0;
+	}
+    }
   else
     wbc = python_dont_write_bytecode == AUTO_BOOLEAN_TRUE ? 0 : 1;
 
@@ -2346,11 +2353,22 @@ python executable."),
 
   add_setshow_auto_boolean_cmd ("dont-write-bytecode", no_class,
 				&python_dont_write_bytecode, _("\
-Set whether the Python interpreter should ignore environment variables."), _(" \
-Show whether the Python interpreter showlist ignore environment variables."), _(" \
-When enabled GDB's Python interpreter will ignore any Python related\n	\
-flags in the environment.  This is equivalent to passing `-E' to a\n	\
-python executable."),
+Set whether the Python interpreter should avoid byte-compiling python modules."), _("\
+Show whether the Python interpreter should avoid byte-compiling python modules."), _("\
+When enabled, GDB's embedded Python interpreter won't byte-compile python\n\
+modules.  In order to take effect, this setting must be enabled in an early\n\
+initialization file, i.e. those run via the --early-init-eval-command or\n\
+-eix command line options.  A 'set python dont-write-bytecode on' command\n\
+can also be issued directly from the GDB command line via the\n\
+--early-init-eval-command or -eiex command line options.\n\
+\n\
+This setting defaults to 'auto'.  In this mode, provided the 'python\n\
+ignore-environment' setting is 'off', the environment variable\n\
+PYTHONDONTWRITEBYTECODE is examined to determine whether or not to\n\
+byte-compile python modules.  PYTHONDONTWRITEBYTECODE is considered to be\n\
+off/disabled either when set to the empty string or when the\n\
+environment variable doesn't exist.  All other settings, including those\n\
+which don't seem to make sense, indicate that it's on/enabled."),
 				set_python_dont_write_bytecode,
 				show_python_dont_write_bytecode,
 				&user_set_python_list,