How to determine why process fails in kotlin script - jvm

Question

I am developing a kotlin script which executes code on the platform which it is running on. Platform code is called using this method from the script:
fun exec(command: String, vararg arguments: String, runLive: Boolean = !isDebug): OutputStream {
val allArgs = arguments.joinToString(" ")
if (runLive) {
val process = Runtime.getRuntime().exec(command, arguments)
val exitCode = process.waitFor()
if (exitCode != 0) {
val platformError = String(BufferedInputStream(process.errorStream).readAllBytes(), Charset.defaultCharset())
throw IllegalStateException("Execution of '$command $allArgs' failed with exit code $exitCode!\n$platformError")
}
return process.outputStream
} else {
println("$command $allArgs")
return object : OutputStream() {
override fun write(b: Int) {
println("dummy $b")
}
}
}
}
In the script, I try to get all the tags for a git repository using this call:
exec(command = "git", "tag", runLive = true)
When the command fails, how can process.errorStream be read? The output now is not readable and the script failure says:
java.lang.IllegalStateException: Execution of 'git tag' failed with exit code 1!
at Snap_tag_main.exec(snap-tag.main.kts:75)
at Snap_tag_main.<init>(snap-tag.main.kts:64)

Answer 1

Not all commands use the error stream when displaying errors. The solution is to bring both the input stream as well as the output stream like this:
if (exitCode != 0) {
val platformMessages = """
${String(BufferedInputStream(process.inputStream).readAllBytes(), Charset.defaultCharset())}
${String(BufferedInputStream(process.errorStream).readAllBytes(), Charset.defaultCharset())}
""".trimIndent().trim()
throw IllegalStateException("Execution of '$command $allArgs' failed with exit code $exitCode!\n$platformMessages\n---")
}
The failure message now looks like
java.lang.IllegalStateException: Execution of 'git tag' failed with exit code 1!
usage: git [--version] [--help] [-C <path>] [-c <name>=<value>]
[--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
[-p | --paginate | -P | --no-pager] [--no-replace-objects] [--bare]
[--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
[--super-prefix=<path>] [--config-env=<name>=<envvar>]
<command> [<args>]
These are common Git commands used in various situations:
start a working area (see also: git help tutorial)
clone Clone a repository into a new directory
init Create an empty Git repository or reinitialize an existing one
work on the current change (see also: git help everyday)
add Add file contents to the index
mv Move or rename a file, a directory, or a symlink
restore Restore working tree files
rm Remove files from the working tree and from the index
examine the history and state (see also: git help revisions)
bisect Use binary search to find the commit that introduced a bug
diff Show changes between commits, commit and working tree, etc
grep Print lines matching a pattern
log Show commit logs
show Show various types of objects
status Show the working tree status
grow, mark and tweak your common history
branch List, create, or delete branches
commit Record changes to the repository
merge Join two or more development histories together
rebase Reapply commits on top of another base tip
reset Reset current HEAD to the specified state
switch Switch branches
tag Create, list, delete or verify a tag object signed with GPG
collaborate (see also: git help workflows)
fetch Download objects and refs from another repository
pull Fetch from and integrate with another repository or a local branch
push Update remote refs along with associated objects
'git help -a' and 'git help -g' list available subcommands and some
concept guides. See 'git help <command>' or 'git help <concept>'
to read about a specific subcommand or concept.
See 'git help git' for an overview of the system.
---
at Snap_tag_main.exec(snap-tag.main.kts:82)
at Snap_tag_main.<init>(snap-tag.main.kts:66)