agent: report fs log errors as http errors

This fixes two bugs:

First, FS Logs API endpoint only propagated error back to user if it was
encoded with code, which isn't common.  Other errors get suppressed and
callers get an empty response with 200 error code.  Now, these endpoints
return  a 500 status code along with the error message.

Before
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:47:21 GMT
< Content-Length: 0
<
* Connection #0 to host 127.0.0.1 left intact
```

After
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:48:12 GMT
< Content-Length: 60
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
alloc lookup failed: index error: UUID must be 36 characters
```

Second, we return 400 status code for request validation errors.

Before
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:47:29 GMT
< Content-Length: 22
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
must provide task name
```

After
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 400 Bad Request
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:49:18 GMT
< Content-Length: 22
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
must provide task name
```

command/agent/fs_endpoint.go

@@ -17,12 +17,12 @@ import (
 )
 
 var (
-	allocIDNotPresentErr  = fmt.Errorf("must provide a valid alloc id")
-	fileNameNotPresentErr = fmt.Errorf("must provide a file name")
-	taskNotPresentErr     = fmt.Errorf("must provide task name")
-	logTypeNotPresentErr  = fmt.Errorf("must provide log type (stdout/stderr)")
-	clientNotRunning      = fmt.Errorf("node is not running a Nomad Client")
-	invalidOrigin         = fmt.Errorf("origin must be start or end")
+	allocIDNotPresentErr  = CodedError(400, "must provide a valid alloc id")
+	fileNameNotPresentErr = CodedError(400, "must provide a file name")
+	taskNotPresentErr     = CodedError(400, "must provide task name")
+	logTypeNotPresentErr  = CodedError(400, "must provide log type (stdout/stderr)")
+	clientNotRunning      = CodedError(400, "node is not running a Nomad Client")
+	invalidOrigin         = CodedError(400, "origin must be start or end")
 )
 
 func (s *HTTPServer) FsRequest(resp http.ResponseWriter, req *http.Request) (interface{}, error) {
@@ -273,13 +273,13 @@ func (s *HTTPServer) Logs(resp http.ResponseWriter, req *http.Request) (interfac
 
 	if followStr := q.Get("follow"); followStr != "" {
 		if follow, err = strconv.ParseBool(followStr); err != nil {
-			return nil, fmt.Errorf("failed to parse follow field to boolean: %v", err)
+			return nil, CodedError(400, fmt.Sprintf("failed to parse follow field to boolean: %v", err))
 		}
 	}
 
 	if plainStr := q.Get("plain"); plainStr != "" {
 		if plain, err = strconv.ParseBool(plainStr); err != nil {
-			return nil, fmt.Errorf("failed to parse plain field to boolean: %v", err)
+			return nil, CodedError(400, fmt.Sprintf("failed to parse plain field to boolean: %v", err))
 		}
 	}
 
@@ -295,7 +295,7 @@ func (s *HTTPServer) Logs(resp http.ResponseWriter, req *http.Request) (interfac
 	if offsetString != "" {
 		var err error
 		if offset, err = strconv.ParseInt(offsetString, 10, 64); err != nil {
-			return nil, fmt.Errorf("error parsing offset: %v", err)
+			return nil, CodedError(400, fmt.Sprintf("error parsing offset: %v", err))
 		}
 	}
 
@@ -388,10 +388,13 @@ func (s *HTTPServer) fsStreamImpl(resp http.ResponseWriter,
 			decoder.Reset(httpPipe)
 
 			if err := res.Error; err != nil {
+				code := 500
 				if err.Code != nil {
-					errCh <- CodedError(int(*err.Code), err.Error())
-					return
+					code = int(*err.Code)
 				}
+
+				errCh <- CodedError(code, err.Error())
+				return
 			}
 
 			if _, err := io.Copy(output, bytes.NewReader(res.Payload)); err != nil {

command/agent/fs_endpoint_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	cstructs "github.com/hashicorp/nomad/client/structs"
+	"github.com/hashicorp/nomad/helper/uuid"
 	"github.com/hashicorp/nomad/nomad/mock"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/testutil"
@@ -518,3 +519,24 @@ func TestHTTP_FS_Logs_Follow(t *testing.T) {
 		p.Close()
 	})
 }
+
+func TestHTTP_FS_Logs_PropagatesErrors(t *testing.T) {
+	t.Parallel()
+	httpTest(t, nil, func(s *TestAgent) {
+		path := fmt.Sprintf("/v1/client/fs/logs/%s?type=stdout&task=web&offset=0&origin=end&plain=true",
+			uuid.Generate())
+
+		p, _ := io.Pipe()
+		defer p.Close()
+
+		req, err := http.NewRequest("GET", path, p)
+		require.NoError(t, err)
+		respW := testutil.NewResponseRecorder()
+
+		_, err = s.Server.Logs(respW, req)
+		require.Error(t, err)
+
+		_, ok := err.(HTTPCodedError)
+		require.Truef(t, ok, "expected a coded error but found: %#+v", err)
+	})
+}