[SV 10593] Export variables to $(shell ...) commands

Export all variables, including exported makefile variables, when
invoking a shell for the $(shell ...) function.  If we detect a
recursive variable expansion, silently ignore that variable and do
not export it.  We do print a debug message.

* NEWS: Announce the potential backward-incompatibility.
* doc/make.texi (Shell Function): Document the export behavior.
* src/main.c (main): Add "shell-export" to .FEATURES.
* src/job.h: New function to free struct childbase.
* src/job.c (free_childbase): Implement it; call from free_child.
* src/function.c (func_shell_base): Use target_environment() to
obtain the proper environment for the shell function.
Use free_childbase() to free memory.
(windows32_openpipe): Don't reset the environment: the caller
already provided a proper PATH variable in envp.
* src/variable.c (target_environment): If we detect a recursive
expansion and we're called from func_shell, ignore the variable.
(sync_Path_environment): Simplify and reduce memory allocation.
* tests/scripts/functions/shell: Add tests for this.

1 files changed, 17 insertions, 0 deletions

diff --git a/doc/make.texi b/doc/make.texi
index 943c0941..085e9c24 100644
--- a/doc/make.texi
+++ b/doc/make.texi
@@ -8616,6 +8616,23 @@ using a very strange shell, this has the same result as
 @w{@samp{$(wildcard *.c)}} (as long as at least one @samp{.c} file
 exists).@refill
 
+All variables that are marked as @code{export} will also be passed to the
+shell started by the @code{shell} function.  It is possible to create a
+variable expansion loop: consider this @file{makefile}:
+
+@example
+export HI = $(shell echo hi)
+all: ; @@echo $$HI
+@end example
+
+When @code{make} wants to run the recipe it must add the variable @var{HI} to
+the environment; to do so it must be expanded.  The value of this variable
+requires an invocation of the @code{shell} function, and to invoke it we must
+create its environment.  Since @var{HI} is exported, we need to expand it to
+create its environment.  And so on.  In this obscure case @code{make} will not
+add recursively-expanded variables to the @code{shell} environment rather than
+fail with an error.
+
 @node Guile Function,  , Shell Function, Functions
 @section The @code{guile} Function
 @findex guile