# Synchronization

This chapter is from the book

## Data Races

When writing concurrent applications, it is common to run into what is called a race condition.15 A race condition occurs when two different goroutines try to access the same shared resource.

Consider Listing 13.25. There are two different goroutines. One goroutine inserts values into a map, and the other goroutine ranges over the map and prints the values.

##### Listing 13.25 Two Goroutines Accessing a Shared Map
```// launch a goroutine to
// write data in the map
go func() {
for i := 0; i < 10; i++ {

// loop putting data in the map
data[i] = true
}

// cancel the context
cancel()
}()

// launch a goroutine to
// read data from the map
go func() {
// loop through the map
// and print the keys/values
for k, v := range data {
fmt.Printf("%d: %v\n", k, v)
}
}()```

In Listing 13.26, we use those two goroutines to write a test to assert the map is written to and read from correctly.

##### Listing 13.26 A Passing Testing Without the Race Detector
```func Test_Mutex(t *testing.T) {
t.Parallel()

// create a new cancellable context
// to stop the test when the goroutines
// are finished
ctx := context.Background()
ctx, cancel := context.WithTimeout(ctx, 20*time.Millisecond)
defer cancel()

// create a map to be used
// as a shared resource
data := map[int]bool{}

// launch a goroutine to
// write data in the map
go func() {
for i := 0; i < 10; i++ {

// loop putting data in the map
data[i] = true
}

// cancel the context
cancel()
}()

// launch a goroutine to
// read data from the map
go func() {
// loop through the map
// and print the keys/values
for k, v := range data {
fmt.Printf("%d: %v\n", k, v)
}
}()

// wait for the context to be canceled
<-ctx.Done()

if len(data) != 10 {
t.Fatalf("expected 10 items in the map, got %d", len(data))
}
}```

```\$ go test -v

=== RUN   Test_Mutex
=== PAUSE Test_Mutex
=== CONT  Test_Mutex
--- PASS: Test_Mutex (0.00s)
PASS
ok      demo    0.471s```

`Go Version: go1.19`

A quick glance at the test output, Listing 13.26, would seem to imply that the tests have passed successfully, but this is not the case.

### The Race Detector

A few of the Go commands, such as test and build, have the -race flag exposed. When used, the -race flag tells the Go compiler to create a special version of the binary or test binary that will detect and report race conditions.

If we run the test again, this time with the -race flag, we get a very different result, as shown in Listing 13.27.

##### Listing 13.27 Tests Failing with the Race Detector
```\$ go test -v -race

=== RUN   Test_Mutex
=== PAUSE Test_Mutex
=== CONT  Test_Mutex
--- PASS: Test_Mutex (0.00s)
==================
WARNING: DATA RACE
Read at 0x00c00011c3f0 by goroutine 9:
runtime.mapdelete()
/usr/local/go/src/runtime/map.go:695 +0x46c
demo.Test_Mutex.func2()
./demo_test.go:46 +0x50

Previous write at 0x00c00011c3f0 by goroutine 8:
runtime.mapaccess2_fast64()
/usr/local/go/src/runtime/map_fast64.go:53 +0x1cc
demo.Test_Mutex.func1()
./demo_test.go:32 +0x50

Goroutine 9 (running) created at:
demo.Test_Mutex()
./demo_test.go:43 +0x188
testing.tRunner()
/usr/local/go/src/testing/testing.go:1439 +0x18c
testing.(*T).Run.func1()
/usr/local/go/src/testing/testing.go:1486 +0x44

Goroutine 8 (finished) created at:
demo.Test_Mutex()
./demo_test.go:28 +0x124
testing.tRunner()
/usr/local/go/src/testing/testing.go:1439 +0x18c
testing.(*T).Run.func1()
/usr/local/go/src/testing/testing.go:1486 +0x44
==================
FAIL
exit status 1
FAIL    demo    0.962s```

`Go Version: go1.19`

As you can see from the output, the Go race detector found a race condition in our code.

If we examine the top two entries in a race condition warning, Listing 13.28, it tells us where the two conflicting lines of code are.

##### Listing 13.28 Reading the Race Detector Output
```Read at 0x00c00018204b by goroutine 9:
demo.Test_Mutex.func2()
problem/demo_test.go:46 +0xa5

Previous write at 0x00c00018204b by goroutine 8:
demo.Test_Mutex.func1()
problem/demo_test.go:32 +0x5c```

A read of the shared resource was happening at demo_test.go:46, and a write was happening at demo_test.go:32. We need to synchronize or lock these two goroutines so that they don’t both try to access the shared resource at the same time.

### Most, but Not All

The Go race detector makes a simple guarantee with you (the end user).

A race condition will panic and crash your application. If the race detector finds a race condition, you must fix it.

### Wrapping Up the Race Detector

The race detector is an invaluable tool when developing Go applications. When running tests with the -race flag, you will notice a slowdown in test performance. The race detector has to do a lot of work to track those conditions.

Once identified, the sync package, Listing 13.29, provides a number of ways that you can fix issues.

##### Listing 13.29 The sync Package
```\$ go doc -short sync

type Cond struct{ ... }
func NewCond(l Locker) *Cond
type Locker interface{ ... }
type Map struct{ ... }
type Mutex struct{ ... }
type Once struct{ ... }
type Pool struct{ ... }
type RWMutex struct{ ... }
type WaitGroup struct{ ... }```

`Go Version: go1.19`

### InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

## Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

## Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

### Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

### Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

### Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

### Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

### Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

### Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

## Other Collection and Use of Information

### Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

### Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

### Do Not Track

This site currently does not respond to Do Not Track signals.

## Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

## Children

This site is not directed to children under the age of 13.

## Marketing

Pearson may send or direct marketing communications to users, provided that

• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• Such marketing is consistent with applicable law and Pearson's legal obligations.
• Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
• Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

## Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

## Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

## Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

## Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

## Sharing and Disclosure

Pearson may disclose personal information, as follows:

• As required by law.
• With the consent of the individual (or their parent, if the individual is a minor)
• In response to a subpoena, court order or legal process, to the extent permitted or required by law
• To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
• In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
• To investigate or address actual or suspected fraud or other illegal activities
• To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
• To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
• To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.