cmTimestamp: Support SOURCE_DATE_EPOCH to override current time

See https://reproducible-builds.org/ for why this is good and
https://reproducible-builds.org/specs/source-date-epoch/ for the
definition of this variable.

Help/command/string.rst

@@ -329,6 +329,12 @@ If no explicit ``<format string>`` is given it will default to:
 
 Write a string which can be used as an identifier in C.
 
+.. note::
+
+  If the ``SOURCE_DATE_EPOCH`` environment variable is set,
+  its value will be used instead of the current time.
+  See https://reproducible-builds.org/specs/source-date-epoch/ for details.
+
 UUID
 """"
 

Help/release/dev/SOURCE_DATE_EPOCH.rst

@@ -0,0 +1,5 @@
+SOURCE_DATE_EPOCH
+-----------------
+
+* The :command:`string(TIMESTAMP)` will now honor the ``SOURCE_DATE_EPOCH``
+  environment variable and use its value instead of the current time.

Source/cmTimestamp.cxx

@@ -5,6 +5,7 @@
 #include <cmConfigure.h>
 #include <cstring>
 #include <sstream>
+#include <stdlib.h>
 
 #include "cmSystemTools.h"
 
@@ -12,6 +13,16 @@ std::string cmTimestamp::CurrentTime(const std::string& formatString,
                                      bool utcFlag)
 {
   time_t currentTimeT = time(CM_NULLPTR);
+  std::string source_date_epoch;
+  cmSystemTools::GetEnv("SOURCE_DATE_EPOCH", source_date_epoch);
+  if (!source_date_epoch.empty()) {
+    std::istringstream iss(source_date_epoch);
+    iss >> currentTimeT;
+    if (iss.fail() || !iss.eof()) {
+      cmSystemTools::Error("Cannot parse SOURCE_DATE_EPOCH as integer");
+      exit(27);
+    }
+  }
   if (currentTimeT == time_t(-1)) {
     return std::string();
   }

Tests/RunCMake/string/RunCMakeTest.cmake

@@ -6,6 +6,11 @@ run_cmake(AppendNoArgs)
 run_cmake(Concat)
 run_cmake(ConcatNoArgs)
 
+run_cmake(Timestamp)
+run_cmake(TimestampEmpty)
+run_cmake(TimestampInvalid)
+run_cmake(TimestampInvalid2)
+
 run_cmake(Uuid)
 run_cmake(UuidMissingNamespace)
 run_cmake(UuidMissingNamespaceValue)

Tests/RunCMake/string/Timestamp-result.txt

@@ -0,0 +1 @@
+0

Tests/RunCMake/string/Timestamp-stderr.txt

@@ -0,0 +1 @@
+RESULT=2005-08-07 23:19:49 Sun Aug 05 day=219 wd=0 week=32 %%I=11

Tests/RunCMake/string/Timestamp.cmake

@@ -0,0 +1,3 @@
+set(ENV{SOURCE_DATE_EPOCH} "1123456789")
+string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S %a %b %y day=%j wd=%w week=%U %%I=%I" UTC)
+message("RESULT=${RESULT}")

Tests/RunCMake/string/TimestampEmpty-result.txt

@@ -0,0 +1 @@
+0

Tests/RunCMake/string/TimestampEmpty-stderr.txt

@@ -0,0 +1 @@
+RESULT=2

Tests/RunCMake/string/TimestampEmpty.cmake

@@ -0,0 +1,3 @@
+set(ENV{SOURCE_DATE_EPOCH} "")
+string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S" UTC)
+message("RESULT=${RESULT}")

Tests/RunCMake/string/TimestampInvalid-result.txt

@@ -0,0 +1 @@
+27

Tests/RunCMake/string/TimestampInvalid-stderr.txt

@@ -0,0 +1 @@
+CMake Error: Cannot parse SOURCE_DATE_EPOCH as integer

Tests/RunCMake/string/TimestampInvalid.cmake

@@ -0,0 +1,3 @@
+set(ENV{SOURCE_DATE_EPOCH} "invalid-integer")
+string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S" UTC)
+message("RESULT=${RESULT}")

Tests/RunCMake/string/TimestampInvalid2-result.txt

@@ -0,0 +1 @@
+27

Tests/RunCMake/string/TimestampInvalid2-stderr.txt

@@ -0,0 +1 @@
+CMake Error: Cannot parse SOURCE_DATE_EPOCH as integer

Tests/RunCMake/string/TimestampInvalid2.cmake

@@ -0,0 +1,3 @@
+set(ENV{SOURCE_DATE_EPOCH} "123trailing-garbage")
+string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S" UTC)
+message("RESULT=${RESULT}")