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sqlite3-api.js And Friends

ext/wasm/api/README.md

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sqlite3-api.js And Friends

This is the README for the files sqlite3-*.js and sqlite3-wasm.c. This collection of files is used to build a single-file distribution of the sqlite3 JS/WASM API. It is broken into multiple JS files because:

  1. To facilitate including or excluding certain components for specific use cases. e.g. by removing sqlite3-api-oo1.js if the OO#1 API is not needed.

  2. To facilitate modularizing the pieces for use in different WASM build environments. e.g. the files post-js-*.js are for use with Emscripten's --post-js feature, and nowhere else. As-yet-hypothetical comparable toolchains would necessarily have similar facilities.

  3. Certain components must be in their own standalone files in order to be loaded as JS Workers.

The structure described here is the current state of things, as of this writing, but is not set in stone forever and may change at any time. This doc targets maintainers of this code and those wanting to dive in to the details, not end user.

First off, a pikchr of the proverbial onion:

pikchr
scale = 0.85
D0: dot invis
define matryoshka {
  $rad = $rad + $2
  circle with se at D0.c radius $rad
  text at last.nw $1 below ljust
#  $anchor = last circle.e
}
$rad = 5mm
C0: circle with se at D0.c "sqlite3-api.js" fit
$rad = C0.width / 2
matryoshka("--post-js",5mm)
matryoshka("Emscripten",12mm)
circle with nw at 7mm s of 3mm e of last circle.nw "--pre-js" fit
matryoshka("--extern-pre/post-js",9mm)
matryoshka("sqlite3.js",6mm)

#CX: last circle
right

CW: file with w at 15mm e of 3rd circle.ne "sqlite3.wasm" fit
arrow from CW.w to 3rd circle.ne

CC: circle with e at 1cm w of last circle.w "End User" fit radius 1cm
arrow <- from CC.e to 6th circle.w

Actually, sqlite3.js and --extern-js are the same. The former is what the client sees and the latter is how it looks from a code maintenance point of view.

At the center of the onion is sqlite3-api.js, which gets generated by concatenating the following files together in their listed order:

  • sqlite3-api-prologue.js
    Contains the initial bootstrap setup of the sqlite3 API objects. This is exposed as a bootstrapping function so that the next step can pass in a config object which abstracts away parts of the WASM environment, to facilitate plugging it in to arbitrary WASM toolchains. The bootstrapping function gets removed from the global scope in a later stage of the bootstrapping process.
  • ../common/whwasmutil.js
    A semi-third-party collection of JS/WASM utility code intended to replace much of the Emscripten glue. The sqlite3 APIs internally use these APIs instead of their Emscripten counterparts, in order to be more portable to arbitrary WASM toolchains and help reduce regressions caused by toolchain-level behavioral changes. This API is configurable, in principle, for use with arbitrary WASM toolchains. It is "semi-third-party" in that it was created in order to support this tree but is standalone and maintained together with...
  • ../jaccwabyt/jaccwabyt.js
    Another semi-third-party API which creates bindings between JS and C structs, such that changes to the struct state from either JS or C are visible to the other end of the connection. This is also an independent spinoff project, conceived for the sqlite3 project but maintained separately.
  • sqlite3-api-glue.js
    Invokes functionality exposed by the previous two files to flesh out low-level parts of sqlite3-api-prologue.js. Most of these pieces involve populating the sqlite3.capi.wasm object and creating sqlite3.capi.sqlite3_...() bindings. This file also deletes most global-scope symbols the above files create, effectively moving them into the scope being used for initializing the API.
  • <build>/sqlite3-api-build-version.js
    Gets created by the build process and populates the sqlite3.version object. This part is not critical, but records the version of the library against which this module was built.
  • sqlite3-api-oo1.js
    Provides a high-level object-oriented wrapper to the lower-level C API, colloquially known as OO API #1. Its API is similar to other high-level sqlite3 JS wrappers and should feel relatively familiar to anyone familiar with such APIs. It is not a "required component" and can be elided from builds which do not want it.
  • sqlite3-api-worker1.js
    A Worker-thread-based API which uses OO API #1 to provide an interface to a database which can be driven from the main Window thread via the Worker message-passing interface. Like OO API #1, this is an optional component, offering one of any number of potential implementations for such an API.
    • sqlite3-worker1.js
      Is not part of the amalgamated sources and is intended to be loaded by a client Worker thread. It loads the sqlite3 module and runs the Worker #1 API which is implemented in sqlite3-api-worker1.js.
    • sqlite3-worker1-promiser.js
      Is likewise not part of the amalgamated sources and provides a Promise-based interface into the Worker #1 API. This is a far user-friendlier way to interface with databases running in a Worker thread.
  • sqlite3-vfs-helper.c-pp.js
    Installs the sqlite3.vfs namespace, which contain helpers for use by downstream code which creates sqlite3_vfs implementations.
  • sqlite3-vtab-helper.c-pp.js
    Installs the sqlite3.vtab namespace, which contain helpers for use by downstream code which creates sqlite3_module implementations.
  • sqlite3-vfs-opfs.c-pp.js
    is an sqlite3 VFS implementation which supports the Origin-Private FileSystem (OPFS) as a storage layer to provide persistent storage for database files in a browser. It requires...
    • sqlite3-opfs-async-proxy.js
      is the asynchronous backend part of the OPFS proxy. It speaks directly to the (async) OPFS API and channels those results back to its synchronous counterpart. This file, because it must be started in its own Worker, is not part of the amalgamation.
  • sqlite3-vfs-opfs-sahpool.c-pp.js
    is another sqlite3 VFS supporting the OPFS, but uses a completely different approach than the above-listed one.

The previous files do not immediately extend the library. Instead they install a global function sqlite3ApiBootstrap(), which downstream code must call to configure the library for the current JS/WASM environment. Each file listed above pushes a callback into the bootstrapping queue, to be called as part of sqlite3ApiBootstrap(). Some files also temporarily create global objects in order to communicate their state to the files which follow them. Those get cleaned up vi post-js-footer.js, described below.

Adapting the build for non-Emscripten toolchains essentially requires packaging the above files, concatated together, into that toolchain's "JS glue" and, in the final stage of that glue, call sqlite3ApiBootstrap() and return its result to the end user.

Files with the extension .c-pp.js are intended to be processed with c-pp, noting that such preprocessing may be applied after all of the relevant files are concatenated. The .c-pp.js extension is used primarily to keep the code maintainers cognisant of the fact that those files contain constructs which may not run as-is in any given JavaScript environment.

The build process glues those files together, resulting in sqlite3-api.js, which is everything except for the Emscripten-specific files detailed below (into which sqlite3-api.js gets injected).

The non-JS outlier file is sqlite3-wasm.c: it is a proxy for sqlite3.c which #include's that file and adds a handful of WASM-specific helper functions, at least two of which requires access to private/static sqlite3.c internals. sqlite3.wasm is compiled from this file rather than sqlite3.c.

The following Emscripten-specific files are injected into the build-generated sqlite3.js along with sqlite3-api.js.

  • extern-pre-js.js
    Emscripten-specific header for Emscripten's --extern-pre-js flag. As of this writing, that file is only used for experimentation purposes and holds no code relevant to the production deliverables.

  • pre-js.c-pp.js
    Emscripten-specific header for Emscripten's --pre-js flag. This file overrides certain Emscripten behavior before Emscripten does most of its work.

  • post-js-header.js
    Emscripten-specific header for the --post-js input. It opens up, but does not close, a function used for initializing the library.

  • sqlite3-api.js gets sandwiched between these ↑ two ↓ files.

  • post-js-footer.js
    Emscripten-specific footer for the --post-js input. This closes off the function opened by post-js-header.js. This file cleans up any dangling globals and runs sqlite3ApiBootstrap(). As of this writing, this code ensures that the previous files leave no more than a single global symbol installed - sqlite3InitModule().

  • extern-post-js.c-pp.js
    Emscripten-specific header for Emscripten's --extern-post-js flag. This file is run in the global scope. It overwrites the Emscripten-installed sqlite3InitModule() function with one which first runs the original implementation, then runs the function installed by post-js-header/footer.js to initialize the library, then initializes the asynchronous parts of the sqlite3 module (for example, the OPFS VFS support). Its final step is to return a Promise of the sqlite3 namespace object to the caller of sqlite3InitModule() (the library user).

<a id='c-pp'></a> Preprocessing of Source Files

Certain files in the build require preprocessing to filter in/out parts which differ between vanilla JS, ES6 Modules, and node.js builds. The preprocessor application itself is in c-pp-lite.c and the complete technical details of such preprocessing are maintained in GNUMakefile.