Bootstrap: allow setting build configuration

When constructing the build configuration for the bootstrap
build is just, start with the value of the environment variable
JUST_BUILD_CONF, if set, instead of the empty object. While there,
also propagate SOURCE_DATE_EPOCH from the environment to the build
configuration.

2 files changed, 25 insertions, 4 deletions

diff --git a/INSTALL.md b/INSTALL.md
index 552b1f8d..e43d1f44 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -70,6 +70,17 @@ before to be present as well).
 Additionally, if the environment variable `DEBUG` is set, the second
 bootstrap phase is carried out sequentially rather than in parallel.
 
+Moreover, when constructing the build configuration, the scripts
+starts with the value of the environment variable `JUST_BUILD_CONF` instead
+of the empty object, if this variable is set. One configuration parameter
+is the build enviroment `ENV` that can be used to set an unusual
+value of `PATH`, e.g.,
+``` sh
+env JUST_BUILD_CONF='{"ENV": {"PATH": "/opt/toolchain/bin"}}' python3 ./bin/boostrap.py
+```
+Additionally, if `SOURCE_DATE_EPOCH` is set in the build environment, it
+is forwarded to the build configuration as well.
+
 In any case, the resulting binary is selfcontained and can be moved
 to an appropriate location in `PATH`.
 
diff --git a/bin/bootstrap.py b/bin/bootstrap.py
index 4b83e033..098662d7 100755
--- a/bin/bootstrap.py
+++ b/bin/bootstrap.py
@@ -37,14 +37,24 @@ LOCAL_LINK_DIRS_MODULE = "src/buildtool/main"
 LOCAL_LINK_DIRS_TARGET = "just"
 
 # architecture related configuration (global variables)
+CONF = {}
+if 'JUST_BUILD_CONF' in os.environ:
+    CONF = json.loads(os.environ['JUST_BUILD_CONF'])
+
 ARCHS = {
   'i686':'x86',
   'x86_64':'x86_64',
   'arm':'arm',
   'aarch64':'arm64'
 }
-MACH = platform.machine()
-CONF = ('{"ARCH":"%s"}' % (ARCHS[MACH],)) if MACH in ARCHS else '{}'
+if "ARCH" not in CONF:
+    MACH = platform.machine()
+    if MACH in ARCHS:
+        CONF["ARCH"] = ARCHS[MACH]
+if 'SOURCE_DATE_EPOCH' in os.environ:
+    CONF['SOURCE_DATE_EPOCH'] = int(os.environ['SOURCE_DATE_EPOCH'])
+
+CONF_STRING = json.dumps(CONF)
 
 # relevant directories (global variables)
 
@@ -250,7 +260,7 @@ def bootstrap():
     GRAPH = os.path.join(WRKDIR, "graph.json")
     TO_BUILD = os.path.join(WRKDIR, "to_build.json")
     run([
-        bootstrap_just, "analyse", "-C", CONF_FILE, "-D", CONF, "--dump-graph", GRAPH,
+        bootstrap_just, "analyse", "-C", CONF_FILE, "-D", CONF_STRING, "--dump-graph", GRAPH,
         "--dump-artifacts-to-build", TO_BUILD, MAIN_MODULE, MAIN_TARGET
     ],
         cwd=src_wrkdir)
@@ -265,7 +275,7 @@ def bootstrap():
         cwd=src_wrkdir)
     OUT = os.path.join(WRKDIR, "out")
     run([
-        "./out-boot/%s" % (MAIN_STAGE, ), "install", "-C", CONF_FILE, "-D", CONF, "-o", OUT,
+        "./out-boot/%s" % (MAIN_STAGE, ), "install", "-C", CONF_FILE, "-D", CONF_STRING, "-o", OUT,
         "--local-build-root", LOCAL_ROOT, MAIN_MODULE, MAIN_TARGET
     ],
         cwd=src_wrkdir)