feat: Allow workspace resources to attach multiple agents (#942)

This enables a "kubernetes_pod" to attach multiple agents that
could be for multiple services. Each agent is required to have
a unique name, so SSH syntax is:

`coder ssh <workspace>.<agent>`

A resource can have zero agents too, they aren't required.

coderd/httpmw/workspaceagentparam.go

@@ -0,0 +1,94 @@
+package httpmw
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"github.com/coder/coder/coderd/database"
+	"github.com/coder/coder/coderd/httpapi"
+)
+
+type workspaceAgentParamContextKey struct{}
+
+// WorkspaceAgentParam returns the workspace agent from the ExtractWorkspaceAgentParam handler.
+func WorkspaceAgentParam(r *http.Request) database.WorkspaceAgent {
+	user, ok := r.Context().Value(workspaceAgentParamContextKey{}).(database.WorkspaceAgent)
+	if !ok {
+		panic("developer error: agent middleware not provided")
+	}
+	return user
+}
+
+// ExtractWorkspaceAgentParam grabs a workspace agent from the "workspaceagent" URL parameter.
+func ExtractWorkspaceAgentParam(db database.Store) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+			agentUUID, parsed := parseUUID(rw, r, "workspaceagent")
+			if !parsed {
+				return
+			}
+			agent, err := db.GetWorkspaceAgentByID(r.Context(), agentUUID)
+			if errors.Is(err, sql.ErrNoRows) {
+				httpapi.Write(rw, http.StatusNotFound, httpapi.Response{
+					Message: "agent doesn't exist with that id",
+				})
+				return
+			}
+			if err != nil {
+				httpapi.Write(rw, http.StatusInternalServerError, httpapi.Response{
+					Message: fmt.Sprintf("get agent: %s", err),
+				})
+				return
+			}
+			resource, err := db.GetWorkspaceResourceByID(r.Context(), agent.ResourceID)
+			if err != nil {
+				httpapi.Write(rw, http.StatusInternalServerError, httpapi.Response{
+					Message: fmt.Sprintf("get resource: %s", err),
+				})
+				return
+			}
+
+			job, err := db.GetProvisionerJobByID(r.Context(), resource.JobID)
+			if err != nil {
+				httpapi.Write(rw, http.StatusInternalServerError, httpapi.Response{
+					Message: fmt.Sprintf("get job: %s", err),
+				})
+				return
+			}
+			if job.Type != database.ProvisionerJobTypeWorkspaceBuild {
+				httpapi.Write(rw, http.StatusBadRequest, httpapi.Response{
+					Message: "Workspace agents can only be fetched for builds.",
+				})
+				return
+			}
+			build, err := db.GetWorkspaceBuildByJobID(r.Context(), job.ID)
+			if err != nil {
+				httpapi.Write(rw, http.StatusInternalServerError, httpapi.Response{
+					Message: fmt.Sprintf("get workspace build: %s", err),
+				})
+				return
+			}
+			workspace, err := db.GetWorkspaceByID(r.Context(), build.WorkspaceID)
+			if err != nil {
+				httpapi.Write(rw, http.StatusInternalServerError, httpapi.Response{
+					Message: fmt.Sprintf("get workspace: %s", err),
+				})
+				return
+			}
+
+			apiKey := APIKey(r)
+			if apiKey.UserID != workspace.OwnerID {
+				httpapi.Write(rw, http.StatusUnauthorized, httpapi.Response{
+					Message: "getting non-personal agents isn't supported",
+				})
+				return
+			}
+
+			ctx := context.WithValue(r.Context(), workspaceAgentParamContextKey{}, agent)
+			next.ServeHTTP(rw, r.WithContext(ctx))
+		})
+	}
+}