Fix Issue 17844 - std.process.execute should allow not capturing stderr

std/process.d

@@ -1430,6 +1430,19 @@ enum Config
     Calling $(LREF wait) or $(LREF kill) with the resulting $(D Pid) is invalid.
     */
     detached = 64,
+
+    /**
+    By default, the $(LREF execute) and $(LREF executeShell) functions
+    will capture child processes' both stdout and stderr. This can be
+    undesirable if the standard output is to be processed or otherwise
+    used by the invoking program, as `execute`'s result would then
+    contain a mix of output and warning/error messages.
+
+    Specify this flag when calling `execute` or `executeShell` to
+    cause invoked processes' stderr stream to be sent to $(REF stderr,
+    std,stdio), and only capture and return standard output.
+    */
+    stderrPassThrough = 128,
 }
 
 
@@ -2484,7 +2497,11 @@ private auto executeImpl(alias pipeFunc, Cmd, ExtraPipeFuncArgs...)(
     import std.array : appender;
     import std.typecons : Tuple;
 
-    auto p = pipeFunc(commandLine, Redirect.stdout | Redirect.stderrToStdout,
+    auto redirect = (config & Config.stderrPassThrough)
+        ? Redirect.stdout
+        : Redirect.stdout | Redirect.stderrToStdout;
+
+    auto p = pipeFunc(commandLine, redirect,
                       env, config, workDir, extraArgs);
 
     auto a = appender!(ubyte[])();
@@ -2546,6 +2563,10 @@ private auto executeImpl(alias pipeFunc, Cmd, ExtraPipeFuncArgs...)(
     auto r3 = executeShell("exit 123");
     assert(r3.status == 123);
     assert(r3.output.empty);
+    auto r4 = executeShell("echo stderr test, please ignore 1>&2",
+        null, Config.stderrPassThrough);
+    assert(r4.status == 0);
+    assert(r4.output.empty);
 }
 
 @safe unittest