Java FFI: Fix build collision when many builds/rules compile the same Java class

src/c_tools.nit

@@ -109,13 +109,18 @@ class ExternFile
 	# Filename relative to the nit-compile folder
 	var filename: String
 
+	# Identifier of the compilation context, used to prevent collisions
+	# between the builds of two parallel jobs
+	var build_context_id: String is writable, noinit
+
 	# The name of the target in the Makefile
 	# Usually the produced .o file
 	fun makefile_rule_name: String is abstract
 
-	# The content of the rule in the make
-	# Usually the one-line shell command after the tabulation
-	fun makefile_rule_content: String is abstract
+	# The lines of the rule in the make
+	#
+	# The shell commands after the tabulation.
+	fun makefile_rule_content: Array[String] is abstract
 
 	fun compiles_to_o_file: Bool do return false
 
@@ -150,7 +155,7 @@ class ExternCFile
 		if not pkgconfigs.is_empty then
 			pkg = "`pkg-config --cflags {pkgconfigs.join(" ")}`"
 		end
-		return "$(CC) $(CFLAGS) -Wall -Wno-unused-function {self.cflags} {pkg} -c -o {o} {ff}"
+		return ["$(CC) $(CFLAGS) -Wall -Wno-unused-function {self.cflags} {pkg} -c -o {o} {ff}"]
 	end
 
 	redef fun compiles_to_o_file do return true

src/compiler/abstract_compiler.nit

@@ -236,7 +236,7 @@ class MakefileToolchain
 		var gc_chooser = new ExternCFile("gc_chooser.c", cc_opt_with_libgc)
 		if cc_opt_with_libgc != "" then gc_chooser.pkgconfigs.add "bdw-gc"
 		compiler.extern_bodies.add(gc_chooser)
-		var clib = toolcontext.nit_dir / "clib"
+		var clib = toolcontext.nit_dir.as(not null) / "clib"
 		compiler.files_to_copy.add "{clib}/gc_chooser.c"
 		compiler.files_to_copy.add "{clib}/gc_chooser.h"
 
@@ -255,6 +255,12 @@ class MakefileToolchain
 			compiler.finalize_ffi_for_module(m)
 		end
 
+		# Prevent collision in the build files of two different parallel jobs
+		# by assigning them a somewhat unique identifier
+		for f in compiler.extern_bodies do
+			f.build_context_id = compiler.realmainmodule.c_name
+		end
+
 		# Copy original .[ch] files to compile_dir
 		for src in compiler.files_to_copy do
 			var basename = src.basename
@@ -537,7 +543,8 @@ endif
 		for f in compiler.extern_bodies do
 			var o = f.makefile_rule_name
 			makefile.write("{o}: {f.filename}\n")
-			makefile.write("\t{f.makefile_rule_content}\n\n")
+			for line in f.makefile_rule_content do makefile.write("\t{line}\n")
+			makefile.write("\n")
 			dep_rules.add(f.makefile_rule_name)
 
 			if f.compiles_to_o_file then ofiles.add(o)

src/ffi/cpp.nit

@@ -181,7 +181,7 @@ class ExternCppFile
 	var mmodule: MModule
 
 	redef fun makefile_rule_name do return "{filename}.o"
-	redef fun makefile_rule_content do return "$(CXX) $(CFLAGS) {mmodule.cppflags[""].join(" ")} -c {filename} -o {filename}.o"
+	redef fun makefile_rule_content do return ["$(CXX) $(CFLAGS) {mmodule.cppflags[""].join(" ")} -c {filename} -o {filename}.o"]
 	redef fun compiles_to_o_file do return true
 end
 

src/ffi/java.nit

@@ -429,7 +429,15 @@ class JavaFile
 	fun full_name: String do return filename.basename(".java")
 
 	redef fun makefile_rule_name do return full_name.replace(".", "/") + ".class"
-	redef fun makefile_rule_content do return "javac {filename} -d ."
+
+	# Compile to a directory limited to this build then atomically move the
+	# result into place, so concurrent builds of the same class (eg. shared
+	# support classes like `nit.app.NitObject`) never load a partial file.
+	redef fun makefile_rule_content do return [
+		"mkdir -p ffi_java_{build_context_id}",
+		"javac -implicit:none {filename} -d ffi_java_{build_context_id}",
+		"mv -f ffi_java_{build_context_id}/{makefile_rule_name} {makefile_rule_name}"]
+
 	redef fun add_to_jar do return true
 end
 

src/ffi/objc.nit

@@ -169,7 +169,7 @@ class ExternObjCFile
 
 	redef fun makefile_rule_name do return "{filename.basename(".m")}_m.o"
 	redef fun makefile_rule_content do
-		return "clang $(CFLAGS) -c {filename} -o {makefile_rule_name}"
+		return ["clang $(CFLAGS) -c {filename} -o {makefile_rule_name}"]
 	end
 	redef fun compiles_to_o_file do return true
 end