Manifest parser

Manifest parser takes an app's or lib's manifest (and arch-specific submanifest like mos_esp8266.yml, if present), and generates an aggregate manifest which contains stuff from the app and all libs. Manifest merging is happening as follows:

• Combine paths and absolutize them properly: sources, includes, filesystem, binary_libs;
• Simply combine other items: modules, libs, config_schema, cflags, cxxflags;
• Expand vars in string maps: build_vars, cdefs (like, if we have a build var FOO somewhere, we can refer to it as ${build_vars.FOO}, and even extend it like FOO: "${build_vars.FOO} bar");
• Narrow the set of supported platforms.
• Combine conds: recursively apply the same to conds, prepending paths etc;

Also, each component's manifest can contain conds, e.g.

conds:
  - when: ${build_vars.FOO} == "1"
    apply:
      build_vars:
        BAR: bar

And these conds should be expanded as late as possible: e.g. we could define build_vars.FOO to be 0 in some lib (and thus by default the BAR won't be defined by the cond above), but then the app can override it to 1, and the BAR should be defined finally.

The key distinction between an arch-specific manifest and a cond with the expression like when: ${mos.platform == "esp8266"} is this: arch-specific manifest is expanded right away after reading the main manifest, but conds (as already mentioned above) need to be expanded as late as possible. This fact has certain implications: e.g. conds can't contain libs. See details below for a thorough explanation.

Details

Let's consider an example: app depends on libA which depends on libB. For the further discussion, assume that libB has this:

build_vars:
  VAR_FROM_LIB_B: from_lib_b

Parsing of the app looks as follows:

Step 1: read all inidividual components' manifests

First of all, we read manifest + arch-specific submanifest of the app, and expressions in arch-specific submanifest are expanded against the app's main manifest:

• app -> app_arch = app_whole

Expressions ${...} which failed to expand at this point are left unexpanded: e.g. submanifest could depend on rootManifest or on some lib, neither of which is available at this point, so those expressions will be expanded later, when we build an aggregate manifest (see below).

Also, for an app manifest, we apply the "last-minute adjustments" given at the command line: platform, build vars, maybe something else (TODO: apply at least cflags and cxxflags as well)

So now, app_whole contains all libs app depends on directly. Now we read each of those libs (which in our case is just libA), and perform the same parsing recursively:

• libA -> libA_arch = libA_whole

And then again recursively form libB:

• libB -> libB_arch = libB_whole

So now we have full manifests for all 3 components, and we build an aggregate manifest:

Step 2: compose aggregate manifest

• rootManifest -> libB_whole -> libA_whole -> app_whole = aggregate

So this time, components can actually use stuff from dependencies: libA can have something like:

build_vars:
  VAR_FROM_LIB_B: ${build_vars.VAR_FROM_LIB_B} and_from_lib_a

which will result in VAR_FROM_LIB_B having the value "from_lib_b and_from_lib_a".

NOTE that instead of applying last-minute adjustments at the step 1, we might actually apply them at this step, so that they just go last in the chain (after app_whole), but then it would require a separate code for the remote build where we only have app manifest.

When this aggregate manifest is ready, conds come into place.

Step 3: expand conds, if any

If aggregate manifest contains no conds, we're done and proceed to the "step 4" with that aggregate manifest. Otherwise (there are some conds), we expand them, and cond "when" expressions are evaluated against the aggregate manifest, but they are expanded into the individual components' manifests: conds in libA_whole expand into libA_whole, conds in app_whole expand into app_whole, etc. It's done this way because everything under conds should be present in aggregate manifest in topological order.

When we expand conds for each component, we go back to step 2 and repeat.

NOTE also that even though cond "when" expressions are evaluated against the aggregate manifest, the expressions inside of the conditionally-applied manifest (like ${build_vars.FOO} bar) are expanded against the individual components' manifests. It wouldn't make sense to expand those against aggregate, because it would result in repetitive insertion of the same values: consider the case when an app has a build var FOO set to "${build_vars.FOO} from_app", and libA has a cond (e.g. if the platform is esp32) which sets FOO to "${build_vars.FOO} from_lib_a". So on esp32, FOO should end up having a value " from_lib_a from_app". On the first run of "step 2", FOO in the aggregate manifest gets the value " from_app". That aggregate manifest has a cond, so we expand it, and if we evaluate expression build_vars.FOO against aggregate manifest, it will be " from_app". Thus, FOO in libA will be " from_app from_lib_a". Now we proceed to the second run of "step 2", and FOO in the aggregate manifest ends up having a value " from_app from_lib_a from_app". It doesn't make sense. Thus, expressions like ${build_vars.FOO} are expanded against individual components' manifests, not aggregate manifest.

Step 4: add sources or binary libs

Actually, the aggregate manifest at this point doesn't have aggregate sources from all components. First, we don't want to add all deps' sources when building a lib, and second, for some lib we might want to add a prebuilt binary instead of sources.

So at this step, we resolve all libs' sources (by "resolve" I mean that we convert paths like /path/to/dir and globs like /path/to/dir/*.foo into absolute paths to concrete files), and depending on preferPrebuiltLibs flag, whether prebuilt binary exists and whether any source code exists, we add to the aggregate manifest either sources or prebuilt binary.

That's basically all.

From the description above, we can conclude a few more limitations:

Limitations

LIMITATION_1 is as follows:

So in the example above, libB defines a build var VAR_FROM_LIB_B, so that "upper" components, like libA or app, can refer to it with ${build_vars.VAR_FROM_LIB_B}.

Now, consider the case when libB defines that build var under some cond, like this:

conds:
  - when: ${mos.platform} == "esp32"
    apply:
      build_vars:
        VAR_FROM_LIB_B: from_lib_b

Now, if libA has this:

build_vars:
  VAR_FROM_LIB_B: ${build_vars.VAR_FROM_LIB_B} and_from_lib_a

It will complain that it can't evaluate build_vars.VAR_FROM_LIB_B. This is because, while we're building aggregate manifest (rootManifest -> libB_whole -> libA_whole .....), libB doesn't yet contain that build var: it's located in a non-expanded cond. So to make it work, libA should override that build var under a cond as well:

conds:
  - when: ${mos.platform} == "esp32"
    apply:
      build_vars:
        VAR_FROM_LIB_B: ${build_vars.VAR_FROM_LIB_B} and_from_lib_a

So, to summarize: we can use stuff from dependencies on the same conds level. If some var is defined at the root level (not under any conds), we can use it in the root level as well (or deeper). If the var is defined in the first conds level, we should use it minimum in the first conds level (or deeper). Etc.

And no, we can't just expand conds earlier, because, as was mentioned in the beginning, one of the goals is to expand conds as late as possible, so that app can override things these conds depend on.

TODO: probably we can try to apply the same workaround as we do when expanding arch-specific submanifest: if we can't evaluate the expression, postpone the evaluation for later, when we might have expanded some conds. However, this might add more confusion: like, if some lib has two levels of conds:

conds:
  - when: ....
    apply:
      build_vars:
        SOME_VAR: 1
      conds:
        - when: ....
          apply:
            build_vars:
              SOME_VAR: ${build_vars.SOME_VAR} 2

and in the app we use that SOME_VAR on a root level like this:

build_vars:
  SOME_VAR: ${build_vars.SOME_VAR} app

Then in the end SOME_VAR will end up being 1 app 2, which is confusing. So, not sure whether it's a good idea in this case.

LIMITATION_2 is as follows:

Conds can't contain libs. At the time we expand conds, we don't look at libs anymore. BUT, even if we do, it would make LIMITATION_1 even more confusing:

Consider that libA contains a cond which adds one more library libC if the platform is esp8266. Then, cond levels for that libC will be shifted: what is root level for libC, will be the level 1 for all the rest of the components. Omg.

One more possible workaround would be to check cond expressions right away, and if it's only about platform, then expand it immediately, unlike the rest of the conds. However, in my opinion this would only add more confusion about what expands when.