docs/devel: testing: Document writing portable test cases

Update the best practices of how to write portable test cases that
can be built and run successfully on both Linux and Windows hosts.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220927110632.1973965-55-bmeng.cn@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/devel/testing.rst

@@ -81,6 +81,36 @@ QTest cases can be executed with
 
    make check-qtest
 
+Writing portable test cases
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Both unit tests and qtests can run on POSIX hosts as well as Windows hosts.
+Care must be taken when writing portable test cases that can be built and run
+successfully on various hosts. The following list shows some best practices:
+
+* Use portable APIs from glib whenever necessary, e.g.: g_setenv(),
+  g_mkdtemp(), g_mkdir().
+* Avoid using hardcoded /tmp for temporary file directory.
+  Use g_get_tmp_dir() instead.
+* Bear in mind that Windows has different special string representation for
+  stdin/stdout/stderr and null devices. For example if your test case uses
+  "/dev/fd/2" and "/dev/null" on Linux, remember to use "2" and "nul" on
+  Windows instead. Also IO redirection does not work on Windows, so avoid
+  using "2>nul" whenever necessary.
+* If your test cases uses the blkdebug feature, use relative path to pass
+  the config and image file paths in the command line as Windows absolute
+  path contains the delimiter ":" which will confuse the blkdebug parser.
+* Use double quotes in your extra QEMU commmand line in your test cases
+  instead of single quotes, as Windows does not drop single quotes when
+  passing the command line to QEMU.
+* Windows opens a file in text mode by default, while a POSIX compliant
+  implementation treats text files and binary files the same. So if your
+  test cases opens a file to write some data and later wants to compare the
+  written data with the original one, be sure to pass the letter 'b' as
+  part of the mode string to fopen(), or O_BINARY flag for the open() call.
+* If a certain test case can only run on POSIX or Linux hosts, use a proper
+  #ifdef in the codes. If the whole test suite cannot run on Windows, disable
+  the build in the meson.build file.
+
 QAPI schema tests
 ~~~~~~~~~~~~~~~~~