tracing: misc follow-ups to 22902

- mention 'Lost X events' workaround
- clarify flush tracepoint docs
- fix typo in tracepoint context
- clarify flush for prune
    The documentation and examples for the `fFlushForPrune` argument
    of the utxocache flush tracepoint weren't clear without looking
    at the code.

    See these comments: https://github.com/bitcoin/bitcoin/pull/22902#issuecomment-987094612

- doc: note that there can be temporary UTXO caches
    Bitcoin Core uses temporary clones of it's _main_ UTXO cache in some
    places. The utxocache:add and :spent tracepoints are triggered when
    temporary caches are changed too. This is documented.

1 files changed, 22 insertions, 10 deletions

diff --git a/doc/tracing.md b/doc/tracing.md
index 68de376043..6ec6a6c218 100644
--- a/doc/tracing.md
+++ b/doc/tracing.md
@@ -110,22 +110,31 @@ Arguments passed:
 
 ### Context `utxocache`
 
+The following tracepoints cover the in-memory UTXO cache. UTXOs are, for example,
+added to and removed (spent) from the cache when we connect a new block.
+**Note**: Bitcoin Core uses temporary clones of the _main_ UTXO cache
+(`chainstate.CoinsTip()`). For example, the RPCs `generateblock` and
+`getblocktemplate` call `TestBlockValidity()`, which applies the UTXO set
+changes to a temporary cache. Similarly, mempool consistency checks, which are
+frequent on regtest, also apply the the UTXO set changes to a temporary cache.
+Changes to the _main_ UTXO cache and to temporary caches trigger the tracepoints.
+We can't tell if a temporary cache or the _main_ cache was changed.
+
 #### Tracepoint `utxocache:flush`
 
-Is called *after* the caches are flushed depending on the mode
-`CChainState::FlushStateToDisk` is called with.
+Is called *after* the in-memory UTXO cache is flushed.
 
 Arguments passed:
-1. Duration in microseconds as `int64`
+1. Time it took to flush the cache microseconds as `int64`
 2. Flush state mode as `uint32`. It's an enumerator class with values `0`
    (`NONE`), `1` (`IF_NEEDED`), `2` (`PERIODIC`), `3` (`ALWAYS`)
-3. Number of coins flushed as `uint64`
-4. Memory usage in bytes as `uint64`
-5. If the flush was pruned as `bool`
+3. Cache size (number of coins) before the flush as `uint64`
+4. Cache memory usage in bytes as `uint64`
+5. If pruning caused the flush as `bool`
 
 #### Tracepoint `utxocache:add`
 
-It is called when a new coin is added to the UTXO cache.
+Is called when a coin is added to a UTXO cache. This can be a temporary UTXO cache too.
 
 Arguments passed:
 1. Transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
@@ -136,7 +145,7 @@ Arguments passed:
 
 #### Tracepoint `utxocache:spent`
 
-It is called when a coin is spent from the UTXO cache.
+Is called when a coin is spent from a UTXO cache. This can be a temporary UTXO cache too.
 
 Arguments passed:
 1. Transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
@@ -147,14 +156,17 @@ Arguments passed:
 
 #### Tracepoint `utxocache:uncache`
 
-It is called when the UTXO with the given outpoint is removed from the cache.
+Is called when a coin is purposefully unloaded from a UTXO cache. This
+happens, for example, when we load an UTXO into a cache when trying to accept
+a transaction that turns out to be invalid. The loaded UTXO is uncached to avoid
+filling our UTXO cache up with irrelevant UTXOs.
 
 Arguments passed:
 1. Transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
 2. Output index as `uint32`
 3. Block height the coin was uncached, as `uint32`
 4. Value of the coin as `int64`
-. If the coin is a coinbase as `bool`
+5. If the coin is a coinbase as `bool`
 
 ## Adding tracepoints to Bitcoin Core