app/vmagent/remotewrite: follow-up for e3a756d82869f8c357b072f6e635ebfc7d65dd2c

- Document the fix
- Move the detection of VictoriaMetrics remoteWrite protocol from client.init() to newHTTPClient()
  This simplifies the fix to the following diff:

diff --git a/app/vmagent/remotewrite/client.go b/app/vmagent/remotewrite/client.go
index 099899c19..70b904af4 100644
--- a/app/vmagent/remotewrite/client.go
+++ b/app/vmagent/remotewrite/client.go
@@ -151,10 +151,6 @@ func newHTTPClient(argIdx int, remoteWriteURL, sanitizedURL string, fq *persiste
        }
        c.sendBlock = c.sendBlockHTTP

-       return c
-}
-
-func (c *client) init(argIdx, concurrency int, sanitizedURL string) {
        useVMProto := forceVMProto.GetOptionalArg(argIdx)
        usePromProto := forcePromProto.GetOptionalArg(argIdx)
        if useVMProto && usePromProto {
@@ -173,6 +169,10 @@ func (c *client) init(argIdx, concurrency int, sanitizedURL string) {
        }
        c.useVMProto = useVMProto

+       return c
+}
+
+func (c *client) init(argIdx, concurrency int, sanitizedURL string) {

app/vmagent/remotewrite/client.go

@@ -147,19 +147,12 @@ func newHTTPClient(argIdx int, remoteWriteURL, sanitizedURL string, fq *persiste
 	}
 	c.sendBlock = c.sendBlockHTTP
 
-	return c
-}
-
-func (c *client) init(argIdx, concurrency int, sanitizedURL string) {
-	// custom proto encoding is supported only over http connections
-	isHTTPClient := c.hc != nil
-
 	useVMProto := forceVMProto.GetOptionalArg(argIdx)
 	usePromProto := forcePromProto.GetOptionalArg(argIdx)
 	if useVMProto && usePromProto {
 		logger.Fatalf("-remoteWrite.useVMProto and -remoteWrite.usePromProto cannot be set simultaneously for -remoteWrite.url=%s", sanitizedURL)
 	}
-	if isHTTPClient && !useVMProto && !usePromProto {
+	if !useVMProto && !usePromProto {
 		// Auto-detect whether the remote storage supports VictoriaMetrics remote write protocol.
 		doRequest := func(url string) (*http.Response, error) {
 			return c.doRequest(url, nil)
@@ -170,10 +163,12 @@ func (c *client) init(argIdx, concurrency int, sanitizedURL string) {
 				"See https://docs.victoriametrics.com/vmagent.html#victoriametrics-remote-write-protocol", sanitizedURL)
 		}
 	}
-	if isHTTPClient {
 	c.useVMProto = useVMProto
+
+	return c
 }
 
+func (c *client) init(argIdx, concurrency int, sanitizedURL string) {
 	if bytesPerSec := rateLimit.GetOptionalArgOrDefault(argIdx, 0); bytesPerSec > 0 {
 		logger.Infof("applying %d bytes per second rate limit for -remoteWrite.url=%q", bytesPerSec, sanitizedURL)
 		c.rl.perSecondLimit = int64(bytesPerSec)

docs/CHANGELOG.md

@@ -19,6 +19,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * FEATURE: [vmctl](https://docs.victoriametrics.com/vmctl.html): `vmctl` `vm-native` mode now splits the migration process on per-metric basis. This allows to migrate metrics one-by-one according to the specified filter. This change allows to retry export/import requests for a specific metric and provides a better understanding of the migration progress. See [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3600).
 * FEATURE: [vmctl](https://docs.victoriametrics.com/vmctl.html): `vmctl` `vm-native` mode now supports `--vm-native-src-headers` and `--vm-native-dst-headers` which defines headers to send with each request to the corresponding source address. Thanks, @baconmania, for the pull request. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3906).
 
+* BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): fix panic when [writing data to Kafka](https://docs.victoriametrics.com/vmagent.html#writing-metrics-to-kafka). The panic has been introduced in [v1.88.0](https://docs.victoriametrics.com/CHANGELOG.html#v1880).
 * BUGFIX: prevent from possible panic during [background merge process](https://docs.victoriametrics.com/#storage). It may occur in rare case and was introduced at [v1.85.0](https://docs.victoriametrics.com/CHANGELOG.html#v1850) when implementing [this feature](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3337).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): stop showing `Please enter a valid Query and execute it` error message on the first load of vmui.
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): support old format of URL params. This should make compatible copying URL between vmui of versions or using `Run in VMUI` button in datasource plugin.