Bug 1723082 - Support reading files > INT32_MAX in IOUtils r=Gijs

Previously, if you attempted to read a file greater than INT32_MAX in size, we
would either fail to read the file (on release) or crash (on debug). This is
due to the implementation of _PR_MD_READ accepting an int32_t of bytes to read
instead of a uint32_t, which nsFileStream::Read accepts and passes on.

Differential Revision: https://phabricator.services.mozilla.com/D131161

dom/chrome-webidl/IOUtils.webidl

@@ -24,7 +24,9 @@
 [ChromeOnly, Exposed=(Window, Worker)]
 namespace IOUtils {
  /**
-   * Reads up to |maxBytes| of the file at |path| according to |opts|.
+   * Reads up to |opts.maxBytes| of the file at |path| according to |opts|.
+   *
+   * NB: The maximum file size that can be read is UINT32_MAX.
    *
    * @param path An absolute file path.
    *
@@ -36,6 +38,8 @@ namespace IOUtils {
    * Reads the UTF-8 text file located at |path| and returns the decoded
    * contents as a |DOMString|.
    *
+   * NB: The maximum file size that can be read is UINT32_MAX.
+   *
    * @param path An absolute file path.
    *
    * @return Resolves with the file contents encoded as a string, otherwise
@@ -46,6 +50,8 @@ namespace IOUtils {
    * Read the UTF-8 text file located at |path| and return the contents
    * parsed as JSON into a JS value.
    *
+   * NB: The maximum file size that can be read is UINT32_MAX.
+   *
    * @param path An absolute path.
    *
    * @return Resolves with the contents of the file parsed as JSON.

dom/docs/ioutils_migration.md

@@ -106,6 +106,9 @@ The following is a detailed comparison with examples of the methods and options
 `IOUtils` provides the following methods to read data from a file. Like
 `OS.File`, they accept an `options` dictionary.
 
+Note: The maximum file size that can be read is `UINT32_MAX` bytes. Attempting
+to read a file larger will result in a `NotReadableError`.
+
 ```idl
 Promise<Uint8Array> read(DOMString path, ...);
 

dom/system/IOUtils.cpp

@@ -896,9 +896,13 @@ Result<IOUtils::JsBuffer, IOUtils::IOError> IOUtils::ReadSync(
     // Read the file from disk.
     uint32_t totalRead = 0;
     while (totalRead != bufSize) {
+      // Read no more than INT32_MAX on each call to stream->Read, otherwise it
+      // returns an error.
+      uint32_t bytesToReadThisChunk =
+          std::min<uint32_t>(bufSize - totalRead, INT32_MAX);
       uint32_t bytesRead = 0;
       if (nsresult rv =
-              stream->Read(toRead.Elements(), bufSize - totalRead, &bytesRead);
+              stream->Read(toRead.Elements(), bytesToReadThisChunk, &bytesRead);
           NS_FAILED(rv)) {
         return Err(IOError(rv).WithMessage(
             "Encountered an unexpected error while reading file(%s)",