https://issues.dlang.org/show_bug.cgi?id=17844

          Issue ID: 17844
           Summary: std.process.execute should allow not capturing stderr
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: dlang-bugzilla@thecybershadow.net

Currently, std.process.execute returns in its output a mix of the executed process's stdout and stderr output.

However, capturing stderr is not always desirable. In fact, unless the D program invoking std.process.execute needs to parse the child process's error messages, or needs to relay the error output of the child process to the user (as it would otherwise not be visible), it is not necessary, and potentially dangerous, as the program might confuse or attempt to parse the program's error output as its produced output.

Compare this to the common shell pattern of redirecting a program's output to a file:

$ program > output.txt

In this manner, should the program fail, the command will complete with a non-zero exit status, and any error messages emitted by the program will be directly visible to the user.

D's execute behaviour is as follows:

$ program > output.txt 2>&1

This mixes output and error streams. If the invoking program or script is not careful to check the error code, and attempts to process the program's output (which now contains error messages mixed in), it can cause such garbage data to propagate further than desired.

This is especially egregious if the child program writes informational messages (warnings) to stderr, which do not affect the correctness of the output (on stdout) or the exit status. In such a case, it becomes impossible to use std.process.execute correctly. In some circumstances, such messages can be printed only in some cases (for example, image processing libraries such as gd, or text encoding conversion libraries such as iconv, will only print warnings to stderr on malformed data), which may cause bugs in D programs only discovered in production.

Thus, std.process.execute needs to be changed to allow stderr to be passed to the invoking D program's stderr, and only capture stdout (probably via a Config flag).

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