RFR: 8339068: [Linux] NPE: Cannot read field "firstFont" because "<local4>" is null

[Jose Pereda]

On Tue, 27 Aug 2024 15:50:54 GMT, Jose Pereda <jpereda at openjdk.org> wrote:

> This PR adds a couple of null checks to `LogicalFont` and `FTFactory`, that make use of `FontConfigManager::getFontConfigFont`, which under certain cases, can return null.
> 
> I've tested this PR on both Linux (Ubuntu 22.04.4) and Android, just using `-Dprism.useFontConfig=false`.
> 
> On Ubuntu, I've manually added some font files to the JDK path `$JAVA_HOME/lib/fonts` (which didn't exist), and the test passes now, though this message is still printed out:
> 
> Error: JavaFX detected no fonts! Please refer to release notes for proper font configuration
> 
> which is confusing to me, because fonts where found in the JDK path after all, and even in the case that there were no fonts found, "the release notes" is an ambiguous reference for the user. 
> 
> Also, instead of adding fonts to the JDK, I tested adding a `logicalfonts.properties` file with `-Dprism.fontdir` and without `fontConfig`, but this case was already working before the patch for this PR, and still works after it.
> 
> Note that if there are no fonts in the JDK path, and `prism.fontdir` is not provided, without `fontConfig`, after this PR, there will still be an exception:
> 
> 
> Caused by: java.lang.NullPointerException: Cannot invoke "com.sun.javafx.font.FontResource.getDefaultAAMode()" because the return value of "com.sun.javafx.font.LogicalFont.getSlot0Resource()" is null
>         at javafx.graphics at 24-internal/com.sun.javafx.font.LogicalFont.getDefaultAAMode(LogicalFont.java:456)
>         at javafx.graphics at 24-internal/com.sun.javafx.font.LogicalFont.getStrike(LogicalFont.java:461)
>         at javafx.graphics at 24-internal/com.sun.javafx.font.PrismFont.getStrike(PrismFont.java:79)
>         at javafx.graphics at 24-internal/com.sun.javafx.text.PrismTextLayout.setContent(PrismTextLayout.java:139)
>         at javafx.graphics at 24-internal/javafx.scene.text.Text.getTextLayout(Text.java:303)
>         at javafx.graphics at 24-internal/javafx.scene.text.Text.needsFullTextLayout(Text.java:258)
>         at javafx.graphics at 24-internal/javafx.scene.text.Text$6.invalidated(Text.java:576)
>  ...
> 
> 
> because `LogicalFont::getSlot0Resource` will return null. Adding null checks after this point will require many changes (`LogicalFont::getSlotResource(0)` is used in many places). Shouldn't we fail gracefully after `LogicalFont::getSlot0Resource` if `slot0FontResource` is finally null?

On Linux, if you use `-Dprism.useFontConfig=false`, but then don't set valid embedded fonts, you get the reported NPE, instead of a different failure, with a more explicit message.
 
But since this property is undocumented, and you indeed need to read the sources to find out about it, you might also discover that setting a fontDir with some fonts and a property file with some properties to use those fonts, will solve the issue and there won't be a NPE anymore.

So I agree that setting this to false, and not supplying fonts is a misconfiguration, but should probably be handled better, with a more explicit message or fast failure.

On Android, however, the situation is different: `fontpath_linux.c`(with the native implementation of `FontConfigManager::getFontConfig`) is not used, but `FTFactory::getFallbacks` calls it nonetheless. 

Using `-Dprism.useFontConfig=false` prevents the runtime issue (UnsatisfiedLinkError), but then since `FontConfigManager.getFontConfigFont` returns null (note there is an `AndroidFontFinder` class that should manage fonts on Android), then the NPE can't be avoided, unless with add this null check. 

I agree that a null check hides the underlying issue, so prabably we should use `isLinux` instead. `PrismFontFactory` has this gate before calling `FontConfigManager`.

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PR Comment: https://git.openjdk.org/jfx/pull/1546#issuecomment-2320826055