Backport part of #2062 to 2.x branch.

[ppkarwasz]

This backports the part of PR #2062 that concerns spring-projects/spring-boot#33450 to the 2.x branch.

It contains two sets of changes:

• it catches exceptions thrown by property sources and logs them as warnings,
• it adds a PropertiesUtil#removePropertySource to allow Spring Boot to remove its custom property source, when the Spring Environment becomes unavailable at shutdown.

Note that the second change will not be effective until Spring Boot starts using it, which can happen only after a 2.24.0 release of Log4j API.

Remark: the part of PR #2062 that concerns #1799 was not backported, since I believe it to be a Spring Boot problem:

1. Spring Boot should not perform an unchecked cast of o.a.l.l.spi.LoggerContext,
2. Spring Boot should not call LogManager.getContext multiple times, but cache the LoggerContext it received at startup.

Fixes #2453

[ppkarwasz]

I also included a fix for LOG4J2-3618 in this PR, which caused the original test case to fail.

If your prefer to split this fix into a separate PR, I can do it.

[vy]

I agree with Phillip Webb's proposal in LOG4J2-3618:

use equals/hashCode to determine if items can be added, and only use the Comparator for sorting

I am not able to see how this achieves that. I would be in favor of the following instead:

1. Leave this comparator as is (only compare using ranks)
2. Initialize the PropertiesUtil.Environment#sources using new ConcurrentSkipListSet(), i.e., no explicit comparator
3. Find iterations over PropertiesUtil.Environment#sources, and employ the comparator there (i.e., .stream().sorted(Comparator.INSTANCE).forEach(...)) iff an order is needed.

[vy]

Why do we only guard against the particular failure cases reported rather than the entire block? That is, I was expecting:

keys.stream().filter(Objects.nonNull).forEach(key -> {
    try {
       // Old logic goes here verbatim
    } catch (final Exception error) {
      // Report the failure
    }
});
for (final PropertySource source : sources) {
    try {
       // Old logic goes here verbatim
    } catch (final Exception error) {
      // Report the failure
    }
}

[rgoers]

Reporting failures will actually generate bug reports. It is very likely that Log4j will try to load the Spring PropertySource before the Spring environment exists. As I recall that will cause an exception during startup of every Spring Boot application, which obviously is unacceptable.