Not compatible with Alpine Linux (musl-based distro)

[dsheets]

Execution fails on Alpine Linux 3.3 with:

? ldd "/home/opam/.opam/system/bin/fs_test_posix"
    /lib/ld-musl-x86_64.so.1 (0x55c8cceac000)
    libc.musl-x86_64.so.1 => /lib/ld-musl-x86_64.so.1 (0x55c8cceac000)
< `ldd /home/opam/.opam/system/bin/fs_test_posix | sed -ne "/libc.so/s/.*=> \\([^ ]*\\).*/\\1/p"`| head -1
fs_test: internal error, uncaught exception:
         Failure("hd")

This is due to fragile libc version extraction code at

sibylfs_src/fs_test/fs_test.ml

Lines 308 to 351 in ec6d449

	let exec = "ldd" in
	let args = [|bin|] in
	let sed_script = "/libc.so/s/.*=> \\([^ ]*\\).*/\\1/p" in
	if exit_command exec args true (function 1 -> Some false | _ -> None)
	then (* ldd can understand the ELF binary *)
	Libc.System, List.hd (
	read_command
	("`ldd "^bin
	^" | sed -ne "^(sprintf "%S" sed_script)
	^"`"
	^"| head -1"
	)
	)
	else (* ldd can't understand the ELF binary;
	we probably have a different libc... *)
	let musl_ldd = "/lib/x86_64-linux-musl/libc.so" in
	if Sys.file_exists musl_ldd
	then begin
	let musl_args = Array.append [|exec|] args in
	let env = [||] in
	let p = create_process_exec_args musl_ldd musl_args env in
	match waitpid [] p.pid with
	| (_, Unix.WEXITED 0) ->
	let b = Buffer.create 1024 in
	ignore (drain_into_buf p.stdout b);
	eprintf "%s\n%!" (Buffer.contents b);
	let sed = create_process "sed" [|"-ne"; sed_script|] env in
	let sed_oc = Unix.out_channel_of_descr sed.stdin in
	Buffer.output_buffer sed_oc b;
	flush sed_oc;
	Unix.close sed.stdin;
	(match waitpid [] sed.pid with
	| (_, Unix.WEXITED 0) ->
	let cmd = input_line (Unix.in_channel_of_descr sed.stdout) in
	(* TODO: should this check that we did, in fact, extract a
	musl so? *)
	Libc.Musl, List.hd (read_command (cmd^" 2>&1 | head -2 | tail -1"))
	| (_, Unix.WEXITED _) -> failwith "Error: sed exec failed."
	| (_,_) -> failwith "Error: sed exec failed unexpectedly."
	)
	| (_, Unix.WEXITED _) -> failwith "Error: musl libc exec failed."
	| (_, _) -> failwith "Error: musl libc exec failed unexpectedly."
	end
	else failwith "Error: libc detection failure. Submit a report."

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