Silently ignore incompatible cache file

Rather than raising an error when a cache file is made for a greater
Modules version, ignore it silently.

This change helps to have different version of the Modules tool deployed
on the same site.

Fixes #515

NEWS.rst

@@ -72,6 +72,8 @@ Modules 5.4.0 (not yet released)
   :envvar:`__MODULES_LMSTICKYRULE` environment variable. It helps to determine
   if stickiness is satisfied when changing loaded modules. Which was not
   working correctly for virtual modules. (fix issue #506)
+* Silently ignore cache file when it requires a greater Modules version. (fix
+  issue #515)
 
 
 .. _5.3 release notes:

doc/source/design/module-cache.rst

@@ -252,6 +252,10 @@ file is used. They are primarily useful to skip I/O tests when walking through
 the content of a modulepath directory. As these I/O tests are done during the
 cache build process, the options are useless when using cache files.
 
+When cache file magic cookie defines a Modules version greater than the
+current one, the cache file is silently ignored. Raisin error is not useful as
+different version of Modules may be deployed in the same site environment.
+
 When cache file is not in sync
 ------------------------------
 

tcl/cache.tcl.in

@@ -221,8 +221,8 @@ proc readCacheContent {cachefile} {
             {Cache ERROR}
       # check if min version requirement is met
       } elseif {[versioncmp {@MODULES_RELEASE@} $fhvers] <0} {
-         reportInternalBug "Cache file requires at least Modules version\
-            $fhvers" $cachefile {Cache ERROR}
+         reportDebug "Cache file $cachefile requires at least Modules version\
+            $fhvers"
       } else {
          # set file content as result
          set res $fdata

testsuite/modules.30-cache/040-cache-use.exp

@@ -205,7 +205,21 @@ modulefile-content bar/1.0 1664035017 #%Module #%Module"
 close $fid
 testouterr_cmd sh {avail -a -t -o } OK $tserr_avail1
 testouterr_cmd sh {avail -a -t -o  foo} OK $tserr_avail2
-testouterr_cmd sh {load bar} $ans_load1b [cacheerr_minverreq $mp1cache 99]
+testouterr_cmd sh {load bar} $ans_load1 {}
+file delete $mp1cache
+
+# test with a module header with required version below current version
+set fid [open $mp1cache w]
+puts $fid "#%Module5.2
+modulefile-content bar/1.0 1664035017 #%Module #%Module"
+close $fid
+testouterr_cmd sh {avail -a -t -o } OK bar/1.0
+testouterr_cmd sh {avail -a -t -o  foo} OK {}
+set ans [list]
+lappend ans [list set __MODULES_LMALTNAME bar/1.0&as|bar/default&as|bar/latest]
+lappend ans [list set _LMFILES_ $mp1/bar/1.0]
+lappend ans [list set LOADEDMODULES bar/1.0]
+testouterr_cmd sh {load bar} $ans {}
 file delete $mp1cache
 
 # tests with a cache containing element that does not exists anymore in modulepath