env: allow bare --env0-from to round-trip

Support --env0-from reading any environment generated by -0,
irrespective of whether -i is specified.

* src/env.c (set_envvars_from_file): Now that this function
always builds its own vector, support reading entries
without an equals, which are just appended to the existing vector.
* doc/coreutils.texi (env invocation): Adjust accordingly.
* tests/env/env0-from.sh: Likewise.

doc/coreutils.texi

@@ -17573,10 +17573,11 @@ Read NUL-terminated environment entries from @var{file}.  If @var{file}
 is @samp{-}, read from standard input.  The file must either be empty or
 end with a NUL byte.
 
-Without @option{--ignore-environment}, each entry must be a
-@samp{@var{name}=@var{value}} assignment.  Assignments replace inherited
-variables with the same name, and a later assignment in @var{file} takes
-precedence over an earlier one.
+Without @option{--ignore-environment}, entries of the form
+@samp{@var{name}=@var{value}} replace inherited variables with the same
+name, and a later assignment in @var{file} takes precedence over an earlier
+one.  Entries without an @samp{=} byte, including empty entries, are
+appended to the environment without replacing another entry.
 
 With @option{--ignore-environment}, the entries replace the environment
 directly.  Their order and contents are preserved, including duplicate

src/env.c

@@ -346,7 +346,7 @@ read_env_file (char const *file)
   return size;
 }
 
-/* Merge the NUL-delimited assignments in FILE into the environment.
+/* Merge the NUL-delimited entries in FILE into the environment.
    Return true if ENV now owns the environment vector.  */
 static bool
 set_envvars_from_file (struct env_vector *env, char const *file)
@@ -380,10 +380,6 @@ set_envvars_from_file (struct env_vector *env, char const *file)
   for (char *assignment = env0_from_buffer; assignment < end; )
     {
       char *next = assignment + strlen (assignment) + 1;
-      if (! strchr (assignment, '='))
-        error (EXIT_CANCELED, 0,
-               _("invalid variable specification %s in %s"),
-               quoteaf_n (0, assignment), quoteaf_n (1, file));
       ++file_count;
       assignment = next;
     }
@@ -424,17 +420,26 @@ set_envvars_from_file (struct env_vector *env, char const *file)
   for (char *assignment = env0_from_buffer; assignment < end; )
     {
       char *next = assignment + strlen (assignment) + 1;
-      devmsg ("setenv:   %s\n", assignment);
-
-      char **slot = hash_lookup (slot_table, &assignment);
-      if (slot)
-        *slot = assignment;
+      char *eq = strchr (assignment, '=');
+      if (! eq)
+        {
+          devmsg ("append:   %s\n", assignment);
+          merged[merged_count++] = assignment;
+        }
       else
         {
-          merged[merged_count] = assignment;
-          if (! hash_insert (slot_table, &merged[merged_count]))
-            xalloc_die ();
-          ++merged_count;
+          devmsg ("setenv:   %s\n", assignment);
+
+          char **slot = hash_lookup (slot_table, &assignment);
+          if (slot)
+            *slot = assignment;
+          else
+            {
+              merged[merged_count] = assignment;
+              if (! hash_insert (slot_table, &merged[merged_count]))
+                xalloc_die ();
+              ++merged_count;
+            }
         }
       assignment = next;
     }

tests/env/env0-from.sh

@@ -148,13 +148,30 @@ printf %s '' >empty || framework_failure_
 env -i --env0-from=empty -0 >out || fail=1
 compare /dev/null out || fail=1
 
-# The file must end in NUL.  Without -i, every record is an assignment.
+# Without -i, append entries that have no name to look up.  Native Windows
+# rejects such entries since its environment cannot represent them.
+case $host_os in
+  mingw* | windows*)
+    printf 'not-an-assignment\0' >invalid || framework_failure_
+    returns_ 125 env --env0-from=invalid >out 2>err || fail=1
+    printf '\0' >invalid || framework_failure_
+    returns_ 125 env --env0-from=invalid >out 2>err || fail=1
+    ;;
+  *)
+    printf 'A=file1\0opaque\0\0A=file2\0B=new\0' >mixed \
+      || framework_failure_
+    printf 'A=file2\0opaque\0\0B=new\0' >exp || framework_failure_
+    LC_ALL=C sort -z exp >exp-sorted || framework_failure_
+    env -i A=old "$abs_top_builddir/src/env$EXEEXT" \
+      --env0-from=mixed -0 >all || fail=1
+    LC_ALL=C sort -z all >out || framework_failure_
+    compare exp-sorted out || fail=1
+    ;;
+esac
+
+# The file must end in NUL.
 printf 'A=unterminated' >invalid || framework_failure_
 returns_ 125 env --env0-from=invalid >out 2>err || fail=1
-printf 'not-an-assignment\0' >invalid || framework_failure_
-returns_ 125 env --env0-from=invalid >out 2>err || fail=1
-printf '\0' >invalid || framework_failure_
-returns_ 125 env --env0-from=invalid >out 2>err || fail=1
 returns_ 125 env --env0-from=does-not-exist >out 2>err || fail=1
 
 # Invalid -u names are still diagnosed after loading variables from a file.