[lld][WebAssembly] Early error if output file cannot be created.

This matches the behaviour of the ELF driver.

Also move the `createFiles` to be `checkConfig` and report `no input
files` there.   Again this is mostly to match the structure of the ELF
linker better.

Differential Revision: https://reviews.llvm.org/D76960

Author: sbc100

lld/test/wasm/early-exit-for-bad-paths.s

@@ -0,0 +1,22 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+
+# RUN: not wasm-ld %t.o -o does_not_exist/output 2>&1 | \
+# RUN:   FileCheck %s -check-prefixes=NO-DIR-OUTPUT,CHECK
+# RUN: not wasm-ld %t.o -o %s/dir_is_a_file 2>&1 | \
+# RUN:   FileCheck %s -check-prefixes=DIR-IS-OUTPUT,CHECK
+# TODO(sbc): check similar check for -Map file once we add that option
+
+# NO-DIR-OUTPUT: error: cannot open output file does_not_exist/output:
+# DIR-IS-OUTPUT: error: cannot open output file {{.*}}/dir_is_a_file:
+
+# We should exit before doing the actual link. If an undefined symbol error is
+# discovered we haven't bailed out early as expected.
+# CHECK-NOT: undefined_symbol
+
+# RUN: not wasm-ld %t.o -o / 2>&1 | FileCheck %s -check-prefixes=ROOT,CHECK
+# ROOT: error: cannot open output file /
+
+_start:
+    .functype _start () -> ()
+    call undefined_symbol
+    end_function

lld/test/wasm/lto/opt-level.ll

@@ -11,7 +11,7 @@
 ; RUN:   FileCheck --check-prefix=INVALID %s
 ; INVALID: invalid optimization level for LTO: 6
 
-; RUN: not wasm-ld -o %t3 -m elf_x86_64 -e main --lto-O-1 %t.o 2>&1 | \
+; RUN: not wasm-ld -o %t3 -e main --lto-O-1 %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALIDNEGATIVE %s
 ; INVALIDNEGATIVE: invalid optimization level for LTO: 4294967295
 

lld/test/wasm/responsefile.test

@@ -10,11 +10,11 @@ RUN:     FileCheck --check-prefix=INVRSP %s
 INVRSP: invalid response file quoting: foobar
 
 RUN: echo "blah\foo" > %t.rsp
-RUN: not wasm-ld --rsp-quoting=windows @%t.rsp 2>&1 | \
+RUN: not wasm-ld -o a.out --rsp-quoting=windows @%t.rsp 2>&1 | \
 RUN:     FileCheck --check-prefix=WINRSP %s
 WINRSP: error: cannot open blah\foo:
 
 RUN: echo "blah\foo" > %t.rsp
-RUN: not wasm-ld --rsp-quoting=posix @%t.rsp 2>&1 | \
+RUN: not wasm-ld -o a.out --rsp-quoting=posix @%t.rsp 2>&1 | \
 RUN:     FileCheck --check-prefix=POSRSP %s
 POSRSP: error: cannot open blahfoo:

lld/wasm/Driver.cpp

@@ -15,6 +15,7 @@
 #include "Writer.h"
 #include "lld/Common/Args.h"
 #include "lld/Common/ErrorHandler.h"
+#include "lld/Common/Filesystem.h"
 #include "lld/Common/Memory.h"
 #include "lld/Common/Reproduce.h"
 #include "lld/Common/Strings.h"
@@ -304,6 +305,8 @@ void LinkerDriver::createFiles(opt::InputArgList &args) {
       break;
     }
   }
+  if (files.empty() && errorCount() == 0)
+    error("no input files");
 }
 
 static StringRef getEntry(opt::InputArgList &args) {
@@ -728,16 +731,27 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
   errorHandler().errorLimit = args::getInteger(args, OPT_error_limit, 20);
 
   readConfigs(args);
+
+  createFiles(args);
+  if (errorCount())
+    return;
+
   setConfigs();
   checkOptions(args);
+  if (errorCount())
+    return;
 
   if (auto *arg = args.getLastArg(OPT_allow_undefined_file))
     readImportFile(arg->getValue());
 
-  if (!args.hasArg(OPT_INPUT)) {
-    error("no input files");
+  // Fail early if the output file or map file is not writable. If a user has a
+  // long link, e.g. due to a large LTO link, they do not wish to run it and
+  // find that it failed because there was a mistake in their command-line.
+  if (auto e = tryCreateFile(config->outputFile))
+    error("cannot open output file " + config->outputFile + ": " + e.message());
+  // TODO(sbc): add check for map file too once we add support for that.
+  if (errorCount())
     return;
-  }
 
   // Handle --trace-symbol.
   for (auto *arg : args.filtered(OPT_trace_symbol))
@@ -748,10 +762,6 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
 
   createSyntheticSymbols();
 
-  createFiles(args);
-  if (errorCount())
-    return;
-
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table.
   for (InputFile *f : files)