Improve version behavior in unusual situations

Author: sethrj

cmake/FlibcppVersion.cmake

@@ -21,6 +21,14 @@ FlibcppVersion
     ${PROJNAME}_VERSION
     ${PROJNAME}_VERSION_STRING
 
+  The project version string uses PEP-440 semantic versioning, and will look
+  like v0.1.2 if the version is actually a tagged release, or v0.1.3+abcdef if
+  it's not.
+
+  If a non-tagged version is exported, or an untagged shallow git clone is used,
+  it's impossible to determine the version from the tag, so a warning will be
+  issued and the version will be set to 0.0.0.
+
 #]=======================================================================]
 
 function(flibcpp_find_version PROJNAME GIT_VERSION_FILE)
@@ -31,8 +39,8 @@ function(flibcpp_find_version PROJNAME GIT_VERSION_FILE)
   if (_TEXTFILE MATCHES "\\$Format:")
     # Not a "git archive": use live git information
     set(_CACHE_VAR "${PROJNAME}_GIT_DESCRIBE")
-    set(_VERSION_STRING "${${_CACHE_VAR}}")
-    if (NOT _VERSION_STRING)
+    set(_CACHED_VERSION "${${_CACHE_VAR}}")
+    if (NOT _CACHED_VERSION)
       # Building from a git checkout rather than a distribution
       find_package(Git REQUIRED)
       execute_process(
@@ -44,27 +52,38 @@ function(flibcpp_find_version PROJNAME GIT_VERSION_FILE)
         OUTPUT_STRIP_TRAILING_WHITESPACE
       )
       if (NOT _GIT_RESULT EQUAL "0")
-        message(FATAL_ERROR "Failed to get ${PROJNAME} version from git: "
+        message(WARNING "Failed to get ${PROJNAME} version from git: "
           "${_GIT_ERR}")
-      endif()
-      if (NOT _VERSION_STRING)
-        message(FATAL_ERROR "Failed to get ${PROJNAME} version from git: "
+      elseif (NOT _VERSION_STRING)
+        message(WARNING "Failed to get ${PROJNAME} version from git: "
           "git describe returned an empty string")
+      else()
+        # Process description tag: e.g. v0.4.0-2-gc4af497 or v0.4.0
+        string(REGEX MATCH "v([0-9.]+)(-[0-9]+-g([0-9a-f]+))?" _MATCH
+          "${_VERSION_STRING}"
+        )
+        if (_MATCH)
+          set(_VERSION_STRING "${CMAKE_MATCH_1}")
+          if (CMAKE_MATCH_2)
+            # *not* a tagged release
+            set(_VERSION_HASH "${CMAKE_MATCH_3}")
+          endif()
+        endif()
       endif()
-      set("${_CACHE_VAR}" "${_VERSION_STRING}" CACHE INTERNAL
-        "Git description for ${PROJNAME}")
-    endif()
-    # Process description tag: e.g. v0.4.0-2-gc4af497 or v0.4.0
-    string(REGEX MATCH "v([0-9.]+)(-[0-9]+-g([0-9a-f]+))?" _MATCH
-      "${_VERSION_STRING}"
-    )
-    if (_MATCH)
-      set(_VERSION_STRING "${CMAKE_MATCH_1}")
-      if (CMAKE_MATCH_2)
-        # *not* a tagged release
-        set(_VERSION_HASH "${CMAKE_MATCH_3}")
+      if (NOT _VERSION_STRING)
+        execute_process(
+          COMMAND "${GIT_EXECUTABLE}" "log" "-1" "--format=%h" "HEAD"
+          WORKING_DIRECTORY "${CMAKE_CURRENT_LIST_DIR}"
+          OUTPUT_VARIABLE _VERSION_HASH
+          OUTPUT_STRIP_TRAILING_WHITESPACE
+        )
       endif()
+      set(_CACHED_VERSION "${_VERSION_STRING}" "${_VERSION_HASH}")
+      set("${_CACHE_VAR}" "${_CACHED_VERSION}" CACHE INTERNAL
+        "Version string and hash for ${PROJNAME}")
     endif()
+    list(GET _CACHED_VERSION 0 _VERSION_STRING)
+    list(GET _CACHED_VERSION 1 _VERSION_HASH)
   else()
     # First line are decorators, second is hash.
     list(GET _TEXTFILE 0 _TAG)
@@ -82,7 +101,9 @@ function(flibcpp_find_version PROJNAME GIT_VERSION_FILE)
   endif()
 
   if (NOT _VERSION_STRING)
-    message(FATAL_ERROR "Could not determine version for ${PROJNAME}")
+    message(WARNING "Could not determine version number ${PROJNAME}: "
+      "perhaps a non-release archive?")
+    set(_VERSION_STRING "0.0.0")
   endif()
 
   if (_VERSION_HASH)