[Heartbeat] Add context to HTTP body read errors (#25499) (#25509)

* [Heartbeat] Add context to HTTP body read errors

This change adds clarity to HTTP body read errors, wrapping the error
with a more identifiable message. Prior to this, you'd just get the low
level error, which made it hard to know exactly where it came from.

* more error improvements

* Add changelog

(cherry picked from commit dc4e874)

Co-authored-by: Andrew Cholakian <[email protected]>

CHANGELOG.next.asciidoc

@@ -306,6 +306,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fixed excessive memory usage introduced in 7.5 due to over-allocating memory for HTTP checks. {pull}15639[15639]
 - Fixed scheduler shutdown issues which would in rare situations cause a panic due to semaphore misuse. {pull}16397[16397]
 - Fixed TCP TLS checks to properly validate hostnames, this broke in 7.x and only worked for IP SANs. {pull}17549[17549]
+- Add Context to otherwise ambiguous HTTP body read errors. {pull}25499[25499]
 
 *Journalbeat*
 

heartbeat/monitors/active/http/respbody.go

@@ -20,6 +20,7 @@ package http
 import (
 	"crypto/sha256"
 	"encoding/hex"
+	"fmt"
 	"io"
 	"net/http"
 	"strings"
@@ -58,7 +59,7 @@ func processBody(resp *http.Response, config responseConfig, validator multiVali
 	respBody, bodyLenBytes, bodyHash, respErr := readBody(resp, bufferBodyBytes)
 	// If we encounter an error while reading the body just fail early
 	if respErr != nil {
-		return nil, "", reason.IOFailed(respErr)
+		return nil, "", reason.IOFailed(fmt.Errorf("failed reading HTTP response body: %w", respErr))
 	}
 
 	// Run any validations
@@ -114,6 +115,12 @@ func readPrefixAndHash(body io.ReadCloser, maxPrefixSize int) (respSize int, pre
 		n += m
 	}
 
+	// The ErrUnexpectedEOF message can be confusing to users, so we clarify it here
+	if err == io.ErrUnexpectedEOF {
+		return 0, "", "", fmt.Errorf("connection closed unexpectedly: %w", err)
+	}
+
+	// Note that io.EOF is distinct from io.ErrUnexpectedEOF
 	if err != nil && err != io.EOF {
 		return 0, "", "", err
 	}