Bugzilla

Running the crash in the debugger confirms the assumption from [comment 11](https://bugzilla.mozilla.org/show_bug.cgi?id=847347#c11).

Though `Substring` just wraps the already allocated memory here it still checks the length against its [`kMaxCapacity`](https://searchfox.org/mozilla-central/rev/62494de24f8d4aa003b6ff911e025e07a6d7a117/xpcom/string/nsTSubstring.h#1357) member, which is significantly lower (aprox. 1GB) - and issues a `MOZ_RELEASE_ASSERT(CheckCapacity(aLength), "String is too large.");`. Though I can understand the rational behind assuming string allocations to not require explicit OOM error handling, in this case it would be advisable to have it.

A naive approach could be to hand-check the allowed capacity like

```
  // CheckCapacity checks, if the data can fit into a nsTSubstring
  if (!nsTSubstring<char>::CheckCapacity(aDataLen)) {
    return NS_ERROR_OUT_OF_MEMORY;
  }
  auto tmp = Substring(aFileData, aDataLen);
  nsCString encodedData;
  nsresult rv = Base64Encode(tmp, encodedData);
  NS_ENSURE_SUCCESS(rv, rv);
```

but `nsTSubstring<char>::CheckCapacity(aDataLen)` is a protected member of `nsTSubstring`. Probably a cleaner approach would be to add a public static function that checks capacity before constructing the substring object?

Running the crash in the debugger confirms the assumption from [comment 11](https://bugzilla.mozilla.org/show_bug.cgi?id=847347#c11).

Though `Substring` just wraps the already allocated memory here it still checks the length against its [`kMaxCapacity`](https://searchfox.org/mozilla-central/rev/62494de24f8d4aa003b6ff911e025e07a6d7a117/xpcom/string/nsTSubstring.h#1357) member, which is significantly lower (aprox. 1GB) - and issues a `MOZ_RELEASE_ASSERT(CheckCapacity(aLength), "String is too large.");`. Though I can understand the rationale behind assuming string allocations to not require explicit OOM error handling, in this case it would be advisable to have it.

A naive approach could be to hand-check the allowed capacity like

```
  // CheckCapacity checks, if the data can fit into a nsTSubstring
  if (!nsTSubstring<char>::CheckCapacity(aDataLen)) {
    return NS_ERROR_OUT_OF_MEMORY;
  }
  auto tmp = Substring(aFileData, aDataLen);
  nsCString encodedData;
  nsresult rv = Base64Encode(tmp, encodedData);
  NS_ENSURE_SUCCESS(rv, rv);
```

but `nsTSubstring<char>::CheckCapacity(aDataLen)` is a protected member of `nsTSubstring`. Probably a cleaner approach would be to add a public static function that checks capacity before constructing the substring object?