[Relocatable] Follow-up 7: Fix hand-off of bytecode image from header.c to ocamlrun #224
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Previously, caml_attempt_open received a pointer to the string and passed this string to caml_search_exe_in_path. This function allocates a fresh string which was then assigned to that pointer. This interface was a little cryptic to follow - in particular, there were several scenarios in which the result was not being freed. It's also not entirely clear that caml_attempt_open involved a PATH-search, which appears to have been applied in error when CAML_DEBUG_FILE was added.
Bytecode startup includes a check which has no place on Windows.
Makes the behaviour of the Unix and Windows versions of the header equivalent, in particular it means that if argv[0] doesn't describe the executable, more things fail for Cygwin.
If a CRT application (including, therefore, another OCaml program), exec's a bytecode program which uses the executable header, the cbReserved2 and lpReserved2 fields of the STARTUPINFO structure are quietly used by the CRT to pass handle information about open fds. The status checker in the cloexec.ml test has C stubs, which causes ocamltest to compile the bytecode version with -custom. The test is split to have an additional intermediate pure OCaml program which simply repeats either the Unix.execv or Unix.create_process call with the Sys.argv it was applied. In bytecode, ocamltest compiles this program just with -use-runtime, which means it uses the executable header on Windows. Without the change to header.c in this commit, that test begins to fail in bytecode on Windows, because the CRT information about inherited handles is not passed on to the status checker (the HANDLE values will have been inherited, as that's a kernel function, but the CRT structures regarding the fds are not initialised, which is what the checker than looks at). The fix here simplifies the code considerably - rather than initialising a fresh STARTUPINFO structure, we simply use GetStartupInfo to retrieve the one which was used to create the process itself and pass that to CreateProcess - cbReserved2 and lpReserved2 are therefore passed on, and the cloexec.ml test passes again.
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The bytecode executable launcher (stdlib/header.c) can _only_ invoke ocamlrun after it has opened itself in order to find out the name of the runtime to execute. However, once ocamlrun is exec'd, the knowledge of this file was previously lost and if it could not be recovered from argv[0], then execution fails. This new approach, for both Windows and Unix, instead keeps the fd for the bytecode image open and passes its number to ocamlrun as __OCAML_EXEC_FD in the environment. ocamlrun detects this environment variable and uses that fd to load the bytecode image. If this fails, the runtime does _not_ fallback to any other mechanisms. On Windows, it is possible to recover the filename from a HANDLE. It is not portably possible to do this on Unix, so the filename which was opened by the stdlib/header.c is instead appended to the environment variable.
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