Pass context to fetch function

[savel999]

package generator

import "text/template"

var tpl = template.Must(template.New("generated").
    Funcs(template.FuncMap{
        "lcFirst": lcFirst,
    }).
    Parse(`
// Code generated by github.com/vektah/dataloaden, DO NOT EDIT.

package {{.Package}}

import (
    "context"
    "sync"
    "time"

    {{if .KeyType.ImportPath}}"{{.KeyType.ImportPath}}"{{end}}
    {{if .ValType.ImportPath}}"{{.ValType.ImportPath}}"{{end}}
)

// {{.Name}}Config captures the config to create a new {{.Name}}
type {{.Name}}Config struct {
    // Fetch is a method that provides the data for the loader 
    Fetch func(ctx context.Context, keys []{{.KeyType.String}}) ([]{{.ValType.String}}, []error)

    // Wait is how long wait before sending a batch
    Wait time.Duration

    // MaxBatch will limit the maximum number of keys to send in one batch, 0 = not limit
    MaxBatch int
}

// New{{.Name}} creates a new {{.Name}} given a fetch, wait, and maxBatch
func New{{.Name}}(config {{.Name}}Config) *{{.Name}} {
    return &{{.Name}}{
        fetch: config.Fetch,
        wait: config.Wait,
        maxBatch: config.MaxBatch,
    }
}

// {{.Name}} batches and caches requests          
type {{.Name}} struct {
    // this method provides the data for the loader
    fetch func(ctx context.Context, keys []{{.KeyType.String}}) ([]{{.ValType.String}}, []error)

    // how long to done before sending a batch
    wait time.Duration

    // this will limit the maximum number of keys to send in one batch, 0 = no limit
    maxBatch int

    // INTERNAL

    // lazily created cache
    cache map[{{.KeyType.String}}]{{.ValType.String}}

    // the current batch. keys will continue to be collected until timeout is hit,
    // then everything will be sent to the fetch method and out to the listeners
    batch *{{.Name|lcFirst}}Batch

    // mutex to prevent races
    mu sync.Mutex
}

type {{.Name|lcFirst}}Batch struct {
    keys    []{{.KeyType}}
    data    []{{.ValType.String}}
    error   []error
    closing bool
    done    chan struct{}
}

// Load a {{.ValType.Name}} by key, batching and caching will be applied automatically
func (l *{{.Name}}) Load(ctx context.Context, key {{.KeyType.String}}) ({{.ValType.String}}, error) {
    return l.LoadThunk(ctx, key)()
}

// LoadThunk returns a function that when called will block waiting for a {{.ValType.Name}}.
// This method should be used if you want one goroutine to make requests to many
// different data loaders without blocking until the thunk is called.
func (l *{{.Name}}) LoadThunk(ctx context.Context, key {{.KeyType.String}}) func() ({{.ValType.String}}, error) {
    l.mu.Lock()
    if it, ok := l.cache[key]; ok {
        l.mu.Unlock()
        return func() ({{.ValType.String}}, error) {
            return it, nil
        }
    }
    if l.batch == nil {
        l.batch = &{{.Name|lcFirst}}Batch{done: make(chan struct{})}
    }
    batch := l.batch
    pos := batch.keyIndex(ctx, l, key)
    l.mu.Unlock()

    return func() ({{.ValType.String}}, error) {
        <-batch.done

        var data {{.ValType.String}}
        if pos < len(batch.data) {
            data = batch.data[pos]
        }

        var err error
        // its convenient to be able to return a single error for everything
        if len(batch.error) == 1 {
            err = batch.error[0]
        } else if batch.error != nil {
            err = batch.error[pos]
        }

        if err == nil {
            l.mu.Lock()
            l.unsafeSet(key, data)
            l.mu.Unlock()
        }

        return data, err
    }
}

// LoadAll fetches many keys at once. It will be broken into appropriate sized
// sub batches depending on how the loader is configured
func (l *{{.Name}}) LoadAll(ctx context.Context, keys []{{.KeyType}}) ([]{{.ValType.String}}, []error) {
    results := make([]func() ({{.ValType.String}}, error), len(keys))

    for i, key := range keys {
        results[i] = l.LoadThunk(ctx, key)
    }

    {{.ValType.Name|lcFirst}}s := make([]{{.ValType.String}}, len(keys))
    errors := make([]error, len(keys))
    for i, thunk := range results {
        {{.ValType.Name|lcFirst}}s[i], errors[i] = thunk()
    }
    return {{.ValType.Name|lcFirst}}s, errors
}

// LoadAllThunk returns a function that when called will block waiting for a {{.ValType.Name}}s.
// This method should be used if you want one goroutine to make requests to many
// different data loaders without blocking until the thunk is called.
func (l *{{.Name}}) LoadAllThunk(ctx context.Context, keys []{{.KeyType}}) (func() ([]{{.ValType.String}}, []error)) {
    results := make([]func() ({{.ValType.String}}, error), len(keys))
    for i, key := range keys {
        results[i] = l.LoadThunk(ctx, key)
    }
    return func() ([]{{.ValType.String}}, []error) {
        {{.ValType.Name|lcFirst}}s := make([]{{.ValType.String}}, len(keys))
        errors := make([]error, len(keys))
        for i, thunk := range results {
            {{.ValType.Name|lcFirst}}s[i], errors[i] = thunk()
        }
        return {{.ValType.Name|lcFirst}}s, errors
    }
}

// Prime the cache with the provided key and value. If the key already exists, no change is made
// and false is returned.
// (To forcefully prime the cache, clear the key first with loader.clear(key).prime(key, value).)
func (l *{{.Name}}) Prime(key {{.KeyType}}, value {{.ValType.String}}) bool {
    l.mu.Lock()
    var found bool
    if _, found = l.cache[key]; !found {
        {{- if .ValType.IsPtr }}
            // make a copy when writing to the cache, its easy to pass a pointer in from a loop var
            // and end up with the whole cache pointing to the same value.
            cpy := *value
            l.unsafeSet(key, &cpy)
        {{- else if .ValType.IsSlice }}
            // make a copy when writing to the cache, its easy to pass a pointer in from a loop var
            // and end up with the whole cache pointing to the same value.
            cpy := make({{.ValType.String}}, len(value))
            copy(cpy, value)
            l.unsafeSet(key, cpy)
        {{- else }}
            l.unsafeSet(key, value)
        {{- end }}
    }
    l.mu.Unlock()
    return !found
}

// Clear the value at key from the cache, if it exists
func (l *{{.Name}}) Clear(key {{.KeyType}}) {
    l.mu.Lock()
    delete(l.cache, key)
    l.mu.Unlock()
}

func (l *{{.Name}}) unsafeSet(key {{.KeyType}}, value {{.ValType.String}}) {
    if l.cache == nil {
        l.cache = map[{{.KeyType}}]{{.ValType.String}}{}
    }
    l.cache[key] = value
}

// keyIndex will return the location of the key in the batch, if its not found
// it will add the key to the batch
func (b *{{.Name|lcFirst}}Batch) keyIndex(ctx context.Context, l *{{.Name}}, key {{.KeyType}}) int {
    for i, existingKey := range b.keys {
        if key == existingKey {
            return i
        }
    }

    pos := len(b.keys)
    b.keys = append(b.keys, key)
    if pos == 0 {
        go b.startTimer(ctx, l)
    }

    if l.maxBatch != 0 && pos >= l.maxBatch-1 {
        if !b.closing {
            b.closing = true
            l.batch = nil
            go b.end(ctx, l)
        }
    }

    return pos
}

func (b *{{.Name|lcFirst}}Batch) startTimer(ctx context.Context, l *{{.Name}}) {
    time.Sleep(l.wait)
    l.mu.Lock()

    // we must have hit a batch limit and are already finalizing this batch
    if b.closing {
        l.mu.Unlock()
        return
    }

    l.batch = nil
    l.mu.Unlock()

    b.end(ctx, l)
}

func (b *{{.Name|lcFirst}}Batch) end(ctx context.Context, l *{{.Name}}) {
    b.data, b.error = l.fetch(ctx, b.keys)
    close(b.done)
}
`))