[LTP] [PATCH v4] doc: Document TEST macro and state TST_RET/ERR rule LTP-002

[Richard Palethorpe]

There are cases where these variables can be safely used by the
library. However it is a difficult problem to identify these cases
automatically.

Identifying any library code which uses them is relatively easy. In
fact it is easier to automate it than by code review. Because it is
such a boring thing to repeatedly look for and comment on.

If a test library function needs these variables it can recreate its
own private copies.

Reviewed-by: Petr Vorel <pvorel@suse.com>
Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
---

The rest of the patchset was merged. 

V4:
* Remove second TST_EXP_PASS example.

 doc/c-test-api.txt                     | 43 ++++++++++++++++++++++++--
 doc/library-api-writing-guidelines.txt | 14 +++++++++
 2 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/doc/c-test-api.txt b/doc/c-test-api.txt
index dd6c608a4..7caf7d86f 100644
--- a/doc/c-test-api.txt
+++ b/doc/c-test-api.txt
@@ -251,6 +251,9 @@ success. It will check for the return value and reports failure if the return
 value is not equal to 0. The call also sets the 'TST_PASS' variable to 1 if
 the call succeeeded.
 
+As seen above, this and similar macros take optional variadic arguments. These
+begin with a format string and then appropriate values to be formatted.
+
 [source,c]
 -------------------------------------------------------------------------------
 static void test(void)
@@ -291,8 +294,44 @@ static void test(void)
 }
 -------------------------------------------------------------------------------
 
-The 'TST_EXP_FAIL2()' is the same as 'TST_EXP_FAIL()' except the return value
-is expected to be non-negative integer if call passes.
+The 'TST_EXP_FAIL2()' is the same as 'TST_EXP_FAIL()' except the return value is
+expected to be non-negative integer if call passes. These macros build upon the
++TEST()+ macro and associated variables.
+
+[source,c]
+-------------------------------------------------------------------------------
+TEST(socket(AF_INET, SOCK_RAW, 1));
+if (TST_RET > -1) {
+	tst_res(TFAIL, "Created raw socket");
+	SAFE_CLOSE(TST_RET);
+} else if (TST_ERR != EPERM) {
+	tst_res(TFAIL | TTERRNO,
+		"Failed to create socket for wrong reason");
+} else {
+	tst_res(TPASS | TTERRNO, "Didn't create raw socket");
+}
+-------------------------------------------------------------------------------
+
+The +TEST+ macro sets +TST_RET+ to its argument's return value and +TST_ERR+ to
++errno+. The +TTERNO+ flag can be used to print the error number's symbolic
+value.
+
+No LTP library function or macro, except those in 'tst_test_macros.h', will
+write to these variables (rule 'LTP-002'). So their values will not be changed
+unexpectedly.
+
+[source,c]
+-------------------------------------------------------------------------------
+TST_EXP_POSITIVE(wait(&status));
+
+if (!TST_PASS)
+	return;
+-------------------------------------------------------------------------------
+
+If the return value of 'wait' is positive. This macro will print a pass result
+and set +TST_PASS+ appropriately. If the return value is zero or negative, then
+it will print fail.  There are many similar macros to those shown here, please
+see 'tst_test_macros.h'.
 
 [source,c]
 -------------------------------------------------------------------------------
diff --git a/doc/library-api-writing-guidelines.txt b/doc/library-api-writing-guidelines.txt
index a4393fc54..2819d4177 100644
--- a/doc/library-api-writing-guidelines.txt
+++ b/doc/library-api-writing-guidelines.txt
@@ -21,10 +21,24 @@ Don't forget to update docs when you change the API.
 2. C API
 --------
 
+2.1 LTP-001: Sources have tst_ prefix
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 API source code is in headers `include/*.h`, `include/lapi/*.h` (backward
 compatibility for old kernel and libc) and C sources in `lib/*.c`. Files have
 'tst_' prefix.
 
+2.2 LTP-002: TST_RET and TST_ERR are not modified
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The test author is guaranteed that the test API will not modify these
+variables. This prevents silent errors where the return value and
+errno are overwritten before the test has chance to check them.
+
+The macros which are clearly intended to update these variables. That
+is +TEST+ and those in 'tst_test_macros.h'. Are of course allowed to
+update these variables.
+
 3. Shell API
 ------------
 
-- 
2.31.1